Power conversion device

ABSTRACT

A power conversion device is provided which includes a plurality of series circuits each formed of a voltage source and a controlled current source. At least two of said series circuits formed of the voltage source and the controlled current source are connected in parallel. Further, parallel connection points of the series circuits connected in parallel form output terminals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/202,676, filed Aug. 22, 2011, and which application is a 371 NationalStage application of PCT/JP2010/052833, filed Feb. 24, 2010, the entirecontent of which is incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a power conversion device, and moreparticularly to the power conversion device interconnected with athree-phase system through a transformer and a DC transmission systemusing the power conversion system.

BACKGROUND ART

A non-patent document 1 listed below proposes a modular multilevelconverter (MMC) by using a switching device (for example, IGBT:Insulated-gate bipolar transistor) capable of ON/OFF control, as amethod of a power conversion device which can output a high voltagehigher than a breakdown voltage of the switching device.

First, a name of each part of MMC is defined in order to explain acircuit configuration of MMC.

In MMC, a bidirectional chopper circuit shown in FIG. 4 forms a unitconverter.

Each unit converter is connected to an external circuit through at leasttwo terminals. In the present embodiment, the two terminals are calledan x-terminal and a y-terminal, respectively. In addition, a voltage ofthe x-terminal against a standard voltage of the y-terminal is called acell voltage.

If a voltage of an energy storage device 405 of the bidirectionalchopper circuit shown in FIG. 4 is denoted by VC, an available value ofthe cell voltage is two, VC and zero.

In the embodiment, a circuit that cascade-connects the x-terminal andthe y-terminal of one or a plurality of the unit converters is called aconverter arm.

Each converter arm has at least two terminals. In the embodiment, thetwo terminals are called a a-terminal and a b-terminal, respectively. Inaddition, a voltage of the a-terminal against a standard voltage of theb-terminal is called an arm voltage. The arm voltage is a sum of cellvoltages of unit converters included in the converter arm.

Since the arm voltage is the sum of cell voltages, the arm voltagebecomes multiples of a voltage VC of an energy storage device providedin each cell.

In the embodiment, a circuit where one terminal of a first reactor isconnected in series to the b-terminal of a first converter arm, oneterminal of a second reactor is connected in series to the otherterminal of the first reactor, and the a-terminal of a second converterarm is connected in series to the other terminal of the second reactor,is called a leg.

The a-terminal of the first converter arm is called a P-terminal, aconnection point of the two reactors is called a M-terminal of the leg,and the b-terminal of the second converter arm is called a N-terminal ofthe leg. Therefore, each leg has at least three terminals that are theP-terminal, M-terminal and the N-terminal. In addition, in theembodiment, a sum of arm voltages of the two converter arms included inthe leg is called a leg voltage.

Since the leg voltage is a sum of arm voltages, the leg voltage alsobecomes multiples of the voltage VC of the energy storage deviceprovided in each cell.

Next, an explanation will be given of a circuit configuration of MMC.Here, as an example, a three-phase MMC will be described.

The P-terminals of three legs are connected to each other and oneterminal is drawn out from the connection point, similarly, theN-terminals of the three legs are connected to each other and the otherterminal is drawn out from the connection point, then, a three-phase MMCcan be configured. In the embodiment, the drawn out terminal from thethree P-terminals connected to each other is called a positive outputterminal of MMC and the drawn out terminal from the three N-terminalsconnected to each other is called a negative output terminal of MMC.

A DC load can be connected between the positive output terminal and thenegative output terminal of MMC.

A three-phase power system can be connected to three M-terminals of thethree legs. In the embodiment, the three M-terminals of the three legsare generally called a three-phase terminal.

Hereinafter, a brief explanation will be given of an operation of MMC.It is assumed that the three-phase terminal is interconnected with athree-phase power system through a transformer or an interconnectionreactor.

Voltages among three-phase terminals can be controlled by controlling anarm voltage of each converter arm configuring the MMC.

For example, if a system frequency component of a voltage among thethree-phase terminals is controlled to be identical to a frequency andamplitude of a system line voltage and only a phase thereof is slightlydelayed in comparison with that of the system line voltage, an activepower flows into the three-phase MMC from the power system.

In addition, if a system frequency component of a voltage among thethree-phase terminals is controlled to be identical to a frequency andamplitude of a system line voltage and only a phase thereof is slightlyadvanced in comparison with that of the system line voltage, an activepower flows into the power system from the three-phase MMC.

If a system frequency component of a voltage among the three-phaseterminals is controlled to be identical to a frequency and phase of asystem line voltage and only an amplitude thereof is slightly increasedin comparison with that of the system line voltage, an advanced reactivepower is generated between the three-phase MMC and the power system.

If a system frequency component of a voltage among the three-phaseterminals is controlled to be identical to a frequency and phase of asystem line voltage and only an amplitude thereof is slightly decreasedin comparison with that of the system line voltage, a delayed reactivepower is generated between the three-phase MMC and the power system.

However, there are two problems in MMC described later.

The first problem is that a reactor is required for each leg.

In the three-phase MMC, if an energy storage device included in eachunit converter is an ideal DC voltage source and if voltages of allideal DC voltage sources are equal to each other, leg voltages of threelegs can be matched by properly controlling a switching timing of eachunit converter.

However, in the actual MMC, an electrolytic capacitor or a battery isused as the energy storage device.

Since each unit converter operates as a single phase converter, aninstantaneous power flowing into/out from the each unit converter has adouble frequency pulsating component of a frequency of power systemconnected to the three-phase terminals or of a frequency of athree-phase load.

In addition, since MMC transmits and receives a DC power to and from aDC load which is connected between a positive output terminal and anegative output terminal, an instantaneous power flowing into/out fromeach unit converter also has a pulsating power component accompanyingthe transmission and reception of the electric power to and from the DCload.

Therefore, a voltage between both ends of an energy storage device (forexample, an electrolytic capacitor or battery) included in each unitconverter is pulsating, and an instantaneous value of the pulsatingcomponent is different for each leg.

As described above, a leg voltage is multiples of the voltage VC of theenergy storage device included in the leg.

When the voltage VC of the energy storage device is different for eachleg, it is impossible to always and completely match leg voltages ofthree legs even if a switching timing of the unit converter is properlycontrolled.

During a period that the leg voltages of the three legs are not matched,a difference among the leg voltages is absorbed by only two reactorsincluded in each leg.

When an inductance of the two reactors included in each leg is zero, thedifference among the leg voltages is absorbed by only a wiring thatconnects among legs during the period. Since an impedance of the wiringis small, an overcurrent flows into the three legs.

Therefore, a reactor is essential for each leg in order to control theovercurrent.

The second problem is that the reactor becomes large when MMC transmitsa DC power to a DC load.

When MMC transmits a DC power to a DC load, it is required to apply azero-phase DC current to a converter arm of each phase and a reactor.Therefore, a current that the zero-phase DC current is superimposed on anormal-phase/reverse-phase current flows in the reactor.

In this case, a maximum current value becomes large in comparison withthe case that only the normal-phase/reverse-phase current flows in thereactor. In order to prevent a magnetic saturation even in the maximumcurrent value, an increase of iron core cross section of the reactor isrequired, and thereby the reactor becomes large.

In addition, a non-patent document 2 discloses an illustration of DCtransmission system shown in FIG. 35 with one line.

The DC transmission system of FIG. 35 includes three-phase AC powersystems 3100, 3170, a breaker 3124 that is disposed in order todisconnect a DC transmission system 3800 from the three-phase AC powersystems 3100, 3170, a conversion transformer 3805 that transforms an ACvoltage, a three-phase full-bridge power conversion device 3801 thatuses a plurality of semiconductor switching devices, and capacitors3802, 3803 that are connected in parallel with the three-phasefull-bridge power conversion device 3801, and a neutral point 3806 thatis connected to the capacitors 3802, 3803 is grounded. In addition, theDC transmission system includes a DC transmission cable 3807 and a DCreactor 3804 that is connected to the power conversion device 3801 andthe DC transmission cable 3807 in series.

Generally, a DC transmission system transmits electric power from athree-phase AC power system to another three-phase AC power system.

PRIOR ART DOCUMENT Non-Patent Document

-   [Non-patent document 1]-   Makoto Hagiwara, Hirofumi Akagi, “PWM Control and Experiment of    Modular Multilevel Converters”, IEEJ Trans. IA, Vol. 128, No. 7, PP.    957-965 (2008)-   [Non-patent document 2]-   L Ronstrom, M L Hoffstein, R Pajo, M Lahtinen “The Estlink HVDC    Light Transmission System” SECURITY AND RELIABILITY OF ELECTRIC    POWER SYSTEMS, CIGRE Regional Meeting, Jun. 18-20, 2007, Tallinn,    Estonia, 21, rue d'Artois, F-75008 PARIS

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

When various kinds of power conversion devices are interconnected with aspecial high-voltage system, generally, a transformer is disposed inorder to increase/decrease a voltage and to secure an electricinsulation.

With respect to a power conversion device that is interconnected with anelectric power system through a transformer and configured bycascade-connecting unit converters, it is an object of the presentinvention to provide the power conversion device that can eliminate areactor and can reduce in volume and weight.

In addition, in a DC transmission system shown in FIG. 35, ashort-circuiting in a DC zone (hereinafter, referred to as DC line) thatconnects between DC output terminals of respective three-phasefull-bridge power conversion devices 3801, which configure the DCtransmission system 3800 and located on both sides thereof, will beexplained, that is, for example, the short-circuiting at a connectionpoint 3901 between a DC reactor 3804 and a DC transmission cable 3807will be explained, using FIG. 36.

If the connection points 3901 are short-circuited, charges of capacitors3802, 3803 are discharged, and an excessive current flows transiently inthe DC transmission cable 3807. Therefore, there is a possibility thatthe DC transmission cable 3807 is burn out. The discharge paths are apath of capacitor 3802-DC reactor 3804-connection point 3901-DC reactor3804-capacitor 3083 and a path of capacitor 3802-DC reactor 3804-DCtransmission cable 3807-connection point 3901-DC transmission cable3807-DC reactor 3804-capacitor 3083.

The discharge current is suppressed by the DC reactor 3804. However, inorder to increase the suppression effect, an increase of inductance ofthe DC reactor is required, thereby resulting in large size and heavyweight.

In a DC transmission system that converts AC power to DC power once andtransmits it from one AC system to the other AC system, the presentinvention provides a power conversion device and a DC transmissionsystem having a function that prevents an overcurrent when charges aredischarged from a DC capacitor (energy storage device) to a DC line.

Means for Solving the Problem

In order to achieve the foregoing purposes, according to the presentinvention, there is provided a power conversion device comprising aseries circuit of a voltage source and a controlled current source, inwhich at least two series circuits of the voltage source and thecontrolled current source are connected in parallel, and parallelconnection points of the series circuits connected in parallel formoutput terminals.

In addition, in order to achieve the foregoing purposes, according tothe present invention, there is provided a power conversion deviceconfigured by connecting a circuit that star-connects three controlledcurrent sources to respective phases of a three-phase voltage sourcefrom which a neutral point of the three-phase voltage source is drawnout, in which a neutral point of the three controlled current sourcesand the neutral point of the three-phase voltage source form outputterminals.

Furthermore, according to the present invention, there is provided apower conversion device, in which the voltage source contains only adifferential mode (or normal phase/reverse phase) component, and thecontrolled current source transmits and receives electric power to andfrom the voltage source by controlling the differential mode (or normalphase/reverse phase) component and transmits and receives electric powerto and from a load device connected to the output terminals or a powersource by controlling a common mode (or zero-phase) component.

In addition, in order to achieve the foregoing purposes, according tothe present invention, there is provided a power conversion devicecomprising a single-phase or multiphase transformer and a converter arm,in which in the power conversion device, each phase of a primary windingof the single-phase or multiphase transformer forms an input terminal; aneutral point is drawn out from a secondary winding of the single-phaseor multiphase transformer; series circuits of the secondary winding ofthe transformer and the converter arm are connected in parallel; and aparallel connection point of the series circuits and the neutral pointof the secondary winding form output terminals.

In addition, in order to achieve the foregoing purposes, according tothe present invention, there is provided a power conversion deviceconfigured by connecting a three-phase transformer, where a neutralpoint is drawn out from a secondary winding of the three-phasetransformer, and a circuit that star-connects three converter arms torespective phases of the secondary winding, in which each phase of aprimary winding of the three-phase transformer forms an input terminal,and a neutral point of the three converter arms and the neutral point ofthe secondary winding form output terminals.

In addition, in order to achieve the foregoing purposes, according tothe present invention, there is provided a power conversion deviceinterconnected with a three-phase power system through a transformer, inwhich a primary winding of the transformer is connected to thethree-phase power system; a secondary winding of the transformer formsan open winding having six terminals; a first converter group consistingof a circuit that star-connects three converter arms is connected to afirst to a third terminals of the secondary winding; a second convertergroup consisting of a circuit that star-connects other three converterarms is connected to a fourth to a sixth terminals of the secondarywinding; and a neutral point of the first converter group and a neutralpoint of the second converter group form output terminals of the powerconversion device.

In addition, in order to achieve the foregoing purposes, according tothe present invention, there is provided a power conversion deviceinterconnected with a three-phase power system through a transformer, inwhich a primary winding of the transformer is connected to thethree-phase power system; a neutral point of a secondary winding of thetransformer is drawn out, forming four terminals; a circuit thatstar-connects three converter arms is connected to respective phases ofthe secondary winding other than the neutral point; and a neutral pointof the three converter arms and the neutral point of the secondarywinding form output terminals of the power conversion device.

Furthermore, according to the present invention, there is provided apower conversion device, in which the converter arm transmits andreceives an electric power to and from a single-phase or multiphasepower system connected to the primary winding of the transformer bycontrolling a differential mode (normal phase/reverse phase) current,and transmits and receives an electric power to and from a load deviceconnected to the output terminal or a power source by controlling acommon mode (zero-phase) component.

Furthermore, according to the present invention, there is provided apower conversion device, in which the transformer comprises a methodthat makes a magnetomotive force caused by a common mode (zero-phase)current flowing in the secondary winding be substantially zero.

Furthermore, according to the present invention, there is provided apower conversion device, in which the primary winding and the secondarywinding are exchanged with each other.

According to a power conversion device of the present invention, sincean exciting inductance and a leakage inductance of the transformercombine the role of the reactor in MMC of Non-patent document 1, thereactor is unnecessary in the present invention, thereby resulting inreduction in size and weight of the power conversion device.

In addition, in order to achieve the foregoing purposes, according tothe present invention, there is provided a power conversion devicehaving a function to convert AC power of an AC system to DC power, inwhich when a DC line, where the DC power flows, is short-circuited, acurrent flowing in the DC line is controlled by electrically insulatinga DC voltage of an energy storage device of the power conversion devicefrom the DC line.

In addition, in order to achieve the foregoing purposes, according tothe present invention, there is provided a power conversion devicecomprising a transformer, a first converter group consisting of acircuit that star-connects three converter arms and a second convertergroup consisting of a circuit that star-connects other three converterarms, in which a primary winding of the transformer is connected to athree-phase AC power system; a winding structure or a method that makesa magnetomotive force caused by a common mode (zero-phase) currentflowing in the secondary winding be substantially zero is provided andthe secondary winding of the transformer forms an open winding havingsix terminals; the first converter group consisting of the circuit thatstar-connects the three converter arms is connected to a first to athird terminals of the secondary winding; the second converter groupconsisting of the circuit that star-connects the other three converterarms is connected to a fourth to a sixth terminals of the secondarywinding; and a neutral point (star-connected point) of the firstconverter group and a neutral point (star-connected point) of the secondconverter group form output terminals of the power conversion device,and the converter arm has a configuration that cascade-connects one or aplurality of unit chopper cells, and when a DC line of the powerconversion device is short-circuited, a low-side switching deviceconstituting the unit chopper cell is turned ON and a high-sideswitching device constituting the unit chopper cell is turned OFF, inorder to control a current flowing in the DC line.

In addition, in order to achieve the foregoing purposes, according tothe present invention, there is provided a power conversion devicecomprising a transformer, a first converter group consisting of acircuit that star-connects three converter arms and a second convertergroup consisting of a circuit that star-connects other three converterarms, in which a primary winding of the transformer is connected to athree-phase AC power system; a method that makes a magnetomotive forcecaused by a common mode (zero-phase) current flowing in a secondarywinding be substantially zero is provided and the secondary winding ofthe transformer forms an open winding having six terminals; the firstconverter group consisting of the circuit that star-connects the threeconverter arms is connected to a first to a third terminals of thesecondary winding; the second converter group consisting of the circuitthat star-connects the other three converter arms is connected to afourth to a sixth terminals of the secondary winding; and a neutralpoint (star-connected point) of the first converter group and a neutralpoint (star-connected point) of the second converter group form outputterminals of the power conversion device, and the converter arm has aconfiguration that cascade-connects one or a plurality of unitfull-bridge cells, and when a DC line of the power conversion device isshort-circuited, two low-side switching devices constituting the unitfull-bridge cell are turned ON and two high-side switching devicesconstituting the unit full-bridge cell are turned OFF, or the twolow-side switching devices are turned OFF and the two high-sideswitching devices are turned ON, in order to control a current flowingin the DC line.

Furthermore, according to the present invention, there is provided apower conversion device that sets a leakage impedance of the transformerso that when a DC line of the power conversion device isshort-circuited, a short-circuit current flowing into the DC line from athree-phase AC power system becomes smaller than a saturation current ofa switching device constituting the power conversion device during atime until the power conversion device is disconnected from thethree-phase AC power system.

Furthermore, according to the present invention, there is provided apower conversion device that disposes a cooling system of the switchingdevice so that when a DC line of the power conversion device isshort-circuited, a junction temperature of the switching deviceconstituting the power conversion device does not exceed a predeterminedvalue by a short-circuit current flowing into from the three-phase ACpower system during a time until the power conversion device isdisconnected from the three-phase AC power system.

Furthermore, according to the present invention, there is provided apower conversion device, in which when the DC line of the powerconversion device is short-circuited, the power conversion device has afunction to make a sum of voltages of the first converter group and thesecond converter group be substantially equal to a voltage having areverse phase of a voltage of the three-phase AC power system.

In addition, in order to achieve the foregoing purposes, according tothe present invention, there is provided a power conversion deviceconnected to an AC power system and comprising a positive convertergroup, a negative converter group, a positive reactor group and anegative reactor group, in which one end of the positive converter groupforms a positive DC output terminal; the other end of the positiveconverter group is connected in series to one end of the positivereactor group; the other end of the positive reactor group is connectedin series to one end of the negative reactor group; the other end of thenegative reactor group is connected in series to one end of the negativeconverter group; and the other end of the negative converter group formsa negative DC output terminal, and a converter arm of the powerconversion device has a configuration that cascade-connects one or aplurality of unit chopper cells and when a DC line of the powerconversion device is short-circuited, a low-side switching deviceconstituting the unit chopper cell is turned ON and a high-sideswitching device constituting the unit chopper cell is turned OFF, inorder to control a current flowing in the DC line.

In addition, in order to achieve the foregoing purposes, according tothe present invention, there is provided a power conversion deviceconnected to an AC power system and comprising a positive convertergroup, a negative converter group, a positive reactor group and anegative reactor group, in which one end of the positive converter groupforms a positive DC output terminal; the other end of the positiveconverter group is connected in series to one end of the positivereactor group; the other end of the positive reactor group is connectedin series to one end of the negative reactor group; the other end of thenegative reactor group is connected in series to one end of the negativeconverter group; and the other end of the negative converter group formsa negative DC output terminal, and a converter arm of the powerconversion device has a configuration that cascade-connects one or aplurality of unit full-bridge cells, and when a DC line of the powerconversion device is short-circuited, two low-side switching devicesconstituting the unit full-bridge cell are turned ON and two high-sideswitching devices constituting the unit full-bridge cell are turned OFF,or the two low-side switching devices are turned OFF and the twohigh-side switching devices are turned ON, in order to control a currentflowing in the DC line.

Furthermore, according to the present invention, there is provided apower conversion device which is connected to a three-phase AC powersystem.

Furthermore, according to the present invention, there is provided apower conversion device, in which a short-circuiting of an AC outputterminal and a short-circuiting of the DC line are distinctly detected.

Furthermore, according to the present invention, there is provided apower conversion device which includes a current detector in the DC lineof the power conversion device and has a function to determine ashort-circuiting of the DC line if a current value detected by thecurrent detector exceeds a predetermined threshold value.

Furthermore, according to the present invention, there is provided apower conversion device which has a function to distinctly detect theshort-circuiting of the AC output terminal of the power conversiondevice and the short-circuiting of the DC line, and includes a currentdetector for detecting a current status of a converter arm of the powerconversion device, in which the short-circuiting of the DC line isdetermined if a sum of currents of the converter arms for three phasesexceeds a predetermined threshold value.

Furthermore, according to the present invention, there is provided apower conversion device, in which the short-circuiting of the AC outputterminal of the power conversion device is determined based on a factthat a current detected by the current detector set in a primary side orsecondary side of a transformer exceeds a predetermined threshold value.

Furthermore, according to the present invention, there is provided apower conversion device, in which the short-circuiting of the AC outputterminal of the power conversion device is determined based on a factthat a difference between current values detected by a current detectorset in a converter arm of the positive converter group of the powerconversion device and the current detector set in the converter arm ofthe negative converter group of the power conversion device exceeds apredetermined threshold value.

Effects of the Present Invention

According to the present invention, there is provided a power conversiondevice which can eliminate a reactor and is reduced in size and weight.

In addition, in the power conversion device of the present invention, azero-phase DC current flows in the secondary winding of the transformerwhen the power conversion device transmits and receives active power toand from a power system. However, since a magnetomotive force to becaused by the zero-phase DC current becomes zero, a magnetic flux doesnot generated.

In addition, according to the power conversion device and DCtransmission system of the present invention, electric charges of theenergy storage device of the power conversion device are not dischargedwhen the DC line is short-circuited, and an overcurrent does not flow inthe DC transmission cable, accordingly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a first embodiment of the presentinvention;

FIG. 2 is a transformer in the first embodiment of the presentinvention;

FIG. 3 is a unit full-bride converter;

FIG. 4 is a unit bidirectional chopper converter;

FIG. 5 is an example of a reactive power compensation device to whichthe first embodiment of the present invention is applied;

FIG. 6 is a circuit diagram showing a second embodiment of the presentinvention;

FIG. 7 is a transformer in the second embodiment of the presentinvention;

FIG. 8 is a circuit diagram showing a third embodiment of the presentinvention;

FIG. 9 is a transformer in the third embodiment of the presentinvention;

FIG. 10 is a circuit diagram showing a fourth embodiment of the presentinvention;

FIG. 11 is a transformer in the fourth embodiment of the presentinvention;

FIG. 12 is a circuit diagram showing a fifth embodiment of the presentinvention;

FIG. 13 is a transformer in the fifth embodiment of the presentinvention;

FIG. 14 is a phasor diagram in the fifth embodiment of the presentinvention;

FIG. 15 is a brief operation waveform in the fifth embodiment of thepresent invention;

FIG. 16 is an example of a DC transmission system to which the fifthembodiment of the present invention is applied;

FIG. 17 is a circuit diagram showing a sixth embodiment of the presentinvention;

FIG. 18 is a transformer in the sixth embodiment of the presentinvention;

FIG. 19 is a circuit diagram showing a seventh embodiment of the presentinvention;

FIG. 20 is a transformer in the seventh embodiment of the presentinvention;

FIG. 21 is a circuit diagram showing an eighth embodiment of the presentinvention;

FIG. 22 is a transformer in the eighth embodiment of the presentinvention;

FIG. 23 is a circuit diagram showing a ninth embodiment of the presentinvention;

FIG. 24 is a transformer in the ninth embodiment of the presentinvention;

FIG. 25 is a circuit diagram showing a tenth embodiment of the presentinvention;

FIG. 26 is a transformer in the tenth embodiment of the presentinvention;

FIG. 27 is a circuit diagram showing an eleventh embodiment of thepresent invention;

FIG. 28 is a circuit diagram showing a twelfth embodiment of the presentinvention;

FIG. 29 is a transformer in the twelfth embodiment of the presentinvention;

FIG. 30 is a unit chopper cell;

FIG. 31 is a circuit diagram showing a thirteenth embodiment of thepresent invention;

FIG. 32 is a unit full-bridge cell;

FIG. 33 is a circuit diagram showing a fourteenth embodiment of thepresent invention;

FIG. 34 is a circuit diagram showing a fifteenth embodiment of thepresent invention;

FIG. 35 is an illustration of a DC transmission system shown in anon-patent document 16;

FIG. 36 is a circuit diagram of the DC transmission system shown in thenon-patent document 16 when the DC transmission system is grounded;

FIG. 37 is an illustration of a cooling feature according to theembodiments of the present invention; and

FIG. 38 is an equivalent electric circuit showing a thermal circuit ofthe cooling feature of the present invention.

EMBODIMENTS OF THE INVENTION

Hereinafter, explanations for embodiments of the present invention willbe given using drawings.

First Embodiment

An explanation will given of a first embodiment embodying the presentinvention.

A configuration of a power conversion device 101 according to thepresent invention will be explained using FIG. 1.

The power conversion device 101 consists of a transformer 105, apositive converter group 112 and a negative converter group 116.

In the present embodiment, each phase of a three-phase power system 100is called an R-phase, an S-phase and a T-phase. In addition, linevoltages are denoted by VRS, VST and VTR, respectively. Furthermore, acurrent flowing in each phase of the three-phase power system 100 iscalled a system current and denoted by IR, IS and IT.

Next, a configuration of the transformer 105 will be explained usingFIG. 1 and FIG. 2.

The transformer 105 includes nine terminals in total that are an R-phaseterminal 102, an S-phase terminal 103, a T-phase terminal 104, a u-phasepositive terminal 106, a v-phase positive terminal 107, a w-phasepositive terminal 108, a u-phase negative terminal 109, a v-phasenegative terminal 110 and a w-phase negative terminal 111.

FIG. 2 shows a polarity of a magnetomotive force that is generated ineach iron core by each winding of the transformer 105, and a wireconnection of the each winding. The transformer 105 includes iron cores202 to 204, and these iron cores 202 to 204 configure a three-leggedcore. A primary winding 200 is delta-connected, and windings 205, 206and 207 each corresponding to a winding between the R-phase and theS-phase, between the S-phase and the T-phase, and between the T-phaseand the R-phase are wound on iron cores 202, 203 and 204, respectively.The number of winding of each of the windings 205 to 207 issubstantially the same.

A secondary winding 201 includes a u-phase winding 209, a v-phasewinding 209 and a w-phase winding 210. The number of winding of each ofthe windings 208 to 210 is substantially the same.

In the first embodiment, a voltage between both ends of the u-phasewinding 208 is denoted by Vu, a voltage between both ends of the v-phasewinding 209 is denoted by Vv and a voltage between both ends of thew-phase winding 210 is denoted by Vw.

A load device 123 is connected between a positive output terminal 121and a negative output terminal 122 of the power conversion device. Inthe specification, a voltage applied to the load device 123 is denotedby VD and a current flowing in the load device 123 is denoted by ID.

Next, configurations of the positive converter group 112 and thenegative converter group 116 will be explained.

The positive converter group 112 consists of a u-phase positiveconverter arm 113, a v-phase positive converter arm 114 and a w-phasepositive converter arm 115. In addition, the negative converter group116 consists of a u-phase negative converter arm 117, a v-phase negativeconverter arm 118 and a w-phase negative converter arm 119.

Each of the converter arms 113 to 115 and 117 to 119 is provided with ana-terminal and a b-terminal.

In the embodiment, a voltage of the a-terminal against a standardvoltage of the b-terminal is called an arm voltage. In addition, each ofthe converter arms 113 to 115 and 117 to 119 is a circuit thatcascade-connects one or a plurality of unit converters 120.

The a-terminal of the u-phase positive converter arm 113 is connected toa positive output terminal 121, and the b-terminal thereof is connectedto the u-phase positive terminal 106 of the transformer 105. Inaddition, in the embodiment, an arm voltage of the u-phase positiveconverter arm 113 is denoted by VarmuH.

The a-terminal of the v-phase positive converter arm 114 is connected tothe positive output terminal 121, and the b-terminal thereof isconnected to the v-phase positive terminal 107 of the transformer 105.In addition, in the embodiment, an arm voltage of the v-phase positiveconverter arm 114 is denoted by VarmvH.

The a-terminal of the w-phase positive converter arm 115 is connected tothe positive output terminal 121, and the b-terminal thereof isconnected to the w-phase positive terminal 108 of the transformer 105.In addition, in the embodiment, an arm voltage of the w-phase positiveconverter arm 115 is denoted by VarmwH.

The a-terminal of the u-phase negative converter arm 117 is connected tothe u-phase negative terminal 109 of the transformer 105, and theb-terminal thereof is connected to a negative output terminal 122. Inaddition, in the embodiment, the arm voltage of the u-phase negativeconverter arm 117 is denoted by VarmuL.

The a-terminal of the v-phase negative converter arm 118 is connected tothe v-phase negative terminal 110 of the transformer 105, and theb-terminal thereof is connected to the negative output terminal 122. Inaddition, in the embodiment, an arm voltage of the v-phase negativeconverter arm 118 is denoted by VarmvL.

The a-terminal of the w-phase negative converter arm 119 is connected tothe w-phase negative terminal 111 of the transformer 105, and theb-terminal thereof is connected to the negative output terminal 122. Inaddition, in the embodiment, an arm voltage of the w-phase negativeconverter arm 119 is denoted by VarmwL.

In the first embodiment, a sum of VarmuH and VarmuL is described by au-phase arm voltage Varmu. In addition, a sum of VarmvH and VarmvL isdescribed by a v-phase arm voltage Varmv. Similarly, a sum of VarmwH andVarmwL is described by a w-phase arm voltage Varmw.

In addition, in the embodiment, a current flowing in the u-phasepositive converter arm 113 and u-phase negative converter arm 117 isdescribed by a u-phase arm current Iu, a current flowing in the v-phasepositive converter arm 114 and v-phase negative converter arm 118 isdescribed by a v-phase arm current Iv, and a current flowing in thew-phase positive converter arm 115 and w-phase negative converter arm119 is described by a w-phase arm current Iw.

Next, a configuration of a unit converter 120 will be explained, usingFIG. 3 and FIG. 4.

FIG. 3 shows an example of internal configuration of the unit converter120. The unit converter of FIG. 3 is a full-bridge circuit. The unitconverter 120 is a two-terminal circuit having an x-terminal 300 and ay-terminal 301, and consists of a x-phase high-side switching device302, a x-phase low-side switching device 303, a y-phase high-sideswitching device 304, a y-phase low-side switching device 305 and anenergy storage device 306. The switching devices 302 to 305 are anON/OFF control power semiconductor device represented by IGBT. Inaddition, the energy storage device 306 is, for example, a capacitor ora battery. In the embodiment, a voltage of the x-terminal against astandard voltage of the y-terminal is called a cell voltage Vcell of theunit converter.

On the other hand, the unit converter 120 may be configured with abidirectional chopper shown in FIG. 4.

The bidirectional chopper shown in FIG. 4 consists of a high-sideswitching device 403, a low-side switching device 404 and an energystorage device 405. The switching devices 403, 404 are an ON/OFF controlpower semiconductor device represented by IGBT. In addition, the energystorage device 405 is, for example, a capacitor or a battery. In theembodiment, the voltage shown in FIG. 4 is also described by the cellvoltage Vcell.

Next, operations of the power conversion device 101 will be explainedfor three cases below.

(1) A case to supply single-phase AC power or DC power to the loaddevice 123 by receiving active power from the three-phase power system100(2) A case to supply active power to the three-phase power system 100 byreceiving single-phase AC power or DC power from the load device 123(3) A case to transmit and receive reactive power to and from thethree-phase power system 100

Next, an explanation will be given of the operations in the case thatthe power conversion device 101 receives active power from thethree-phase power system 100 and supplies single-phase AC power or DCpower to the load device 123. Here, for example, the following cases aresupposed that the load device 123 is a DC transmission line and thepower conversion device 101 is a power conversion device on the powertransmitting side as seen from the DC transmission line, or the loaddevice 123 is a motor drive inverter and the motor drive inverter isoperated with power running, or the load device 123 is a single-phase ACload.

In the embodiment, voltages of the line voltages VRS, VST and VTR of thethree-phase power system 100 converted to the voltages on the secondaryside of the transformer are denoted by aVRS, aVST and aVTR,respectively. Here, a is a turn ratio of a secondary winding to aprimary winding of the transformer.

Here, an explanation will be given of a relationship among voltages Vu,Vv, Vw of the secondary winding of the transformer, the arm voltagesVarmu, Varmv, Varmw and the voltage VD of the load device 123.

A relationship among Vu, Varmu and VD is expressed by the followingformula.

Vu=VD−Varmu  (Number 1)

A relationship among Vv, Varmv and VD is expressed by the followingformula.

Vv=VD−Varmv  (Number 2)

A relationship among Vw, Varmw and VD is expressed by the followingformula.

Vw=VD−Varmw  (Number 3)

According to the Numbers 1 to 3, the voltages Vu, Vv, Vw of thesecondary winding of the transformer can be controlled by controllingthe u-phase arm voltage Varmu, the v-phase arm voltage Varmv and thew-phase arm voltage Varmw.

Here, the reason why a reactor is unnecessary in the first embodimentwill be explained.

A sum aVRS+Varmu that is the sum of the voltage aVRS, which is thevoltage of the line voltage VRS between the R-phase and the S-phase ofthe three-phase power system 100 converted to the voltage on thesecondary side of the transformer, and the u-phase arm voltage Varmu, asum aVST+Varmv that is the sum of the voltage aVST, which is the voltageof the line voltage VST between the S-phase and the T-phase of thethree-phase power system 100 converted to the voltage on the secondaryside of the transformer, and the v-phase arm voltage Varmv, and a sumaVTR+Varmw that is the sum of the voltage aVTR, which is the voltage ofthe line voltage VTR between the T-phase and the R-phase of thethree-phase power system 100 converted to the voltage on the secondaryside of the transformer, and the u-phase arm voltage Varmw, may bedifferent to each other.

Differences among the aVRS+Varmu, aVST+Varmv and aVTR+Varmw are absorbedby a leakage inductance of the transformer 105.

Then, a reactor is not required in the first embodiment.

If phases of Vu, Vv and Vw are slightly delayed in comparison with thephases of aVRS, aVST and aVTR, while matching frequencies and amplitudesof Vu, Vv and Vw with those of aVRS, aVST and aVTR, an active currentflows into the power conversion device 101 from the three-phase powersystem 100.

Next, an explanation will be given about that the arm voltage can becontrolled by a switching condition of a switching device constitutingthe unit converter 120.

First, an explanation will be given of a case that the unit converter120 is a full-bridge circuit (FIG. 3).

An x-phase high-side switching device 302 and an x-phase low-sideswitching device 303 are alternately switched ON/OFF. In addition, ay-phase high-side switching device 304 and a y-phase low-side switchingdevice 305 are alternately switched ON/OFF.

When the x-phase high-side switching device 302 is ON, the x-phaselow-side switching device 303 is OFF, the y-phase high-side switchingdevice 304 is OFF and the y-phase low-side switching device 305 is ON,the cell voltage Vcell is substantially equal to the voltage VC of theenergy storage device 306 without depending on the current Icell.

When the x-phase high-side switching device 302 is ON, the x-phaselow-side switching device 303 is OFF, the y-phase high-side switchingdevice 304 is ON and the y-phase low-side switching device 305 is OFF,the cell voltage Vcell is substantially zero without depending on thecurrent Icell.

When the x-phase high-side switching device 302 is OFF, the x-phaselow-side switching device 303 is ON, the y-phase high-side switchingdevice 304 is OFF and the y-phase low-side switching device 305 is ON,the cell voltage Vcell is substantially zero without depending on thecurrent Icell.

When the x-phase high-side switching device 302 is OFF, the x-phaselow-side switching device 303 is ON, the y-phase high-side switchingdevice 304 is ON and the y-phase low-side switching device 305 is OFF,the cell voltage Vcell is substantially equal to a voltage that has areverse polarity of the voltage VC of the energy storage device 306without depending on the current Icell.

When the x-phase high-side switching device 302, the x-phase low-sideswitching device 303, the y-phase high-side switching device 304 and they-phase low-side switching device 305 are all OFF, the cell voltageVcell is determined depending on a polarity of the current Icell. Whenthe Icell is positive, the cell voltage Vcell is substantially equal tothe voltage VC of the energy storage device 306. When the Icell isnegative, the cell voltage Vcell is substantially equal to a voltagethat has a reverse polarity of the voltage VC of the energy storagedevice 306.

Next, an explanation will be given of a case that the unit converter 120is a bidirectional chopper (FIG. 4).

When a high-side switching device 403 is ON and a low-side switchingdevice 404 is OFF, the cell voltage Vcell is substantially equal to thevoltage VC of a DC capacitor 405 without depending on the current Icell.

When the high-side switching device 403 is OFF and the low-sideswitching device 404 is ON, the cell voltage Vcell is substantially zerowithout depending on the current Icell.

When the high-side switching device 403 and the low-side switchingdevice 404 are both OFF, the cell voltage Vcell is determined dependingon a polarity of the current Icell. If the Icell is positive, the cellvoltage Vcell is substantially equal to the voltage VC of the energystorage device 405. If the Icell is negative, the cell voltage Vcell issubstantially zero.

Next, an explanation will be given of a method for supplying electricpower to the load device 123.

A current ID flowing in the load device 123 is a sum (Iu+Iv+Iw) of armcurrents Iu, Iv and Iw. When the arm voltages Varmu, Varmv and Varmw donot contain a zero-phase component, the arm currents Iu, Iv and Iw alsodo not contain a zero-phase component. When the arm currents Iu, Iv andIw do not contain the zero-phase component, it becomes thatIu+Iv+Iw=ID=0, and transmission of electric power to the load device 123becomes impossible.

In this case, active power flowing into the power conversion device 101from the three-phase power system 100 is stored in the energy storagedevice (for example, electrolytic capacitor) in each of the respectiveunit converters 120.

In order to supply electric power to the load device 123, the zero-phasecomponent of the arm voltages Varmu, Varmv and Varmw is adjusted and thezero-phase component of the arm currents Iu, Iv and Iw is controlled.According to the Kirchhoff's current law, since it becomes thatID=Iu+Iv+Iw, the current ID can be supplied to the load device 123 byadjusting the zero-phase component of the arm currents Iu, Iv and Iw.

Meanwhile, when active power flowing into the power conversion device101 from the three-phase power system 100 is equal to the active powerconsumed by the load device 123, an amount of energy flowing into/outfrom each unit converter 120 during one cycle of the three-phase powersystem 100 becomes substantially zero.

In addition, as the current ID, direct current, alternate current and acurrent that alternate current is superimposed on direct current may beused.

When the power conversion device 101 and the load device 123 transmitand receive only a single-phase reactive power, active power flowinginto the power conversion device 101 from the three-phase power system100 is controlled to be zero.

Below, an explanation will be given of operations in the case that thepower conversion device 101 receives active power from the load device123 and active power is supplied to the three-phase power system 100.Here, for example, the following cases are supposed that the load device123 is a DC transmission line and the power conversion device 101 is apower conversion device on the power receiving side as seen from the DCtransmission line, or the load device 123 is a motor drive inverter andthe motor drive inverter is operated on the regenerative braking, or theload device 123 is a single-phase AC source.

If only phases of Vu, Vv and Vw are slightly advanced in comparison withthe phases of aVRS, aVST and aVTR, while matching frequencies andamplitudes of Vu, Vv and Vw with those of aVRS, aVST and aVTR, activepower can be supplied to the three-phase power system 100 from the powerconversion device 101.

Next, an explanation will be given of a method for receiving electricpower from the load device 123.

The current ID flowing out from the load device 123 is a sum (Iu+Iv+Iw)of arm currents Iu, Iv and Iw. When the arm voltages Varmu, Varmv andVarmw do not contain a zero-phase component, the arm currents Iu, Iv andIw also do not contain a zero-phase component. When the arm currents Iu,Iv and Iw do not contain the zero-phase component, it becomes thatIu+Iv+Iw=ID=0, and electric power can not be supplied from the loaddevice 123.

In this case, active power flowing into the three-phase power system 100from the power conversion device 101 is supplied from the energy storagedevice (for example, electrolytic capacitor) in each unit converter 120.

In order to have electric power flow into the power conversion device101 from the load device 123, a zero-phase component of the arm voltagesVsrmu, Varmv and Varmw is adjusted and the zero-phase component of thearm currents Iu, Iv and Iw is controlled. According to the Kirchhoff'scurrent law, since it becomes that ID=Iu+Iv+lw, the current ID can besupplied to the power conversion device 101 by adjusting the zero-phasecomponent of the arm currents Iu, Iv and Iw.

Meanwhile, when active power flowing into the three-phase power system100 from the power conversion device 101 is equal to the active powerflowing into the power conversion device 101 from the load device 123,an amount of energy flowing into/out from each unit converter 120 duringone cycle of the three-phase power system 100 is substantially zero.

Below, an explanation will be given of a case that the power conversiondevice 101 transmits and receives reactive power to and from thethree-phase power system 100 and the load device 123 is opened (ID=0).Here, it is supposed that, for example, the power conversion device 101operates as a reactive power compensation device.

If only amplitudes of Vu, Vv and Vw are slightly increased in comparisonwith the amplitudes of aVRS, aVST and aVTR, while matching frequenciesand phases of Vu, Vv and Vw with those of aVRS, aVST and aVTR, advancedreactive power can be supplied to the three-phase power system 100 fromthe power conversion device 101.

In addition, if only amplitudes of Vu, Vv and Vw are slightly decreasedin comparison with the amplitudes of aVRS, aVST and aVTR, while matchingfrequencies and phases of Vu, Vv and Vw with those of aVRS, aVST andaVTR, delayed reactive power can be supplied to the three-phase powersystem 100 from the power conversion device 101.

Next, an explanation will be given about that in the present embodiment,a series circuit of the secondary winding of the transformer and theconverter arm can be considered as a voltage source and a controlledcurrent source.

The three-phase power system 100 is connected to the primary winding ofthe transformer. Since the three-phase power system 100 can beconsidered as a voltage source, a voltage induced in the secondarywinding by the three-phase power system 100 can also be considered as avoltage source.

In addition, the converter arm can adjust a voltage applied to a leakageinductance and exciting inductance of the secondary winding of thetransformer by properly adjusting an arm voltage of the converter arm.

A current flowing in the leakage inductance and the exciting inductanceis proportional to a time integration of the voltage applied to theleakage inductance and the exciting inductance. Therefore, the converterarm can control a current flowing in the leakage inductance and theexciting inductance through the arm voltage of the converter arm.

Therefore, the series circuit of the converter arm and the leakageinductance as well as the exciting inductance can be considered as acontrolled current source.

In the present embodiment, a power conversion device interconnected witha three-phase power system has been described. In a three-phase system,a positive-phase/negative-phase current corresponds to a differentialmode current, and a zero-phase current corresponds to a common current.

In addition, the present embodiment can be applied to a power conversiondevice interconnected with a single-phase or a multiphase system as wellas a three-phase power system by increasing or decreasing the number ofconverter arms.

As an example of application of the present embodiment, the examplewhere the power conversion device 101 is applied to a reactive powercompensation device is shown. FIG. 5 is an example of an electric powersubstation where the power conversion device 101 is installed. Anelectric power substation 501 is interconnected with a three-phase powersystem 500. Loads 503 and the power conversion device 101 according tothe present embodiment are connected to an electric power substationbusbar 502. By properly adjusting a reactive power Q between the powerconversion device 101 and the three-phase power system 500 using theforegoing method, amplitude of a voltage V of the electric powersubstation busbar 502 is controlled to be constant.

Second Embodiment

An explanation will be given of a second embodiment of the presentinvention. In the first embodiment, the primary winding of thetransformer was delta-connected. However, in the second embodiment, theprimary winding of the transformer is star-connected.

Hereinafter, the explanation will be given of only a part ofconfiguration of the second embodiment different from the firstembodiment.

FIG. 6 is a circuit diagram showing the second embodiment of the presentinvention. A power conversion device 600 is interconnected with thethree-phase power system 100 through three-phase AC terminals 102 to104, and transmits and receives active/reactive power to and from thethree-phase power system 100. The power conversion device 600 consistsof a transformer 601, a positive converter group 112 and a negativeconverter group 116 different.

In the embodiment, phase voltages of the R-phase, the S-phase and theT-phase of the three-phase power system 100 are denoted by VR, VS andVT, respectively.

The positive converter group 112 and the negative converter group 116 ofFIG. 6 are identical to those in the first embodiment (FIG. 1).

FIG. 7 shows a polarity of a magnetomotive force that is generated ineach iron core by each winding of the transformer 601, and a wireconnection of the each winding. The transformer 601 includes iron cores202 to 204, and these iron cores 202 to 204 configure a three-leggedcore. A primary winding 700 is star-connected, and windings 701, 702 and703 corresponding to the R-phase, the S-phase and the T-phase,respectively are wound on the iron cores 202, 203 and 204, respectively.

The secondary winding 201 of FIG. 7 is identical to that of thesecondary winding 201 of FIG. 2.

Third Embodiment

An explanation will be given of a third embodiment embodying the presentinvention. The third embodiment is a modification of the firstembodiment. In the first embodiment, two converter groups that are onthe positive side and the negative side are used. However, in the thirdembodiment, only one converter group is used.

In the third embodiment, the number of terminal of the transformer canbe reduced from nine terminals to seven terminals, while effectsidentical to those of the first embodiment can be obtained.

Below, an explanation will be given of only a part of configuration ofthe third embodiment different from the first embodiment.

FIG. 8 is a circuit diagram showing a third embodiment of the presentinvention. A power conversion device 800 is interconnected with thethree-phase power system 100 through three-phase AC terminals 102 to104, and transmits and receives active/reactive power to and from thethree-phase power system 100. The power conversion device 800 consistsof a transformer 801 and a converter group 806.

The transformer 801 includes seven terminals in total that are anR-phase terminal 102, an S-phase terminal 103, a T-phase terminal 104,an u-phase terminal 802, a v-phase terminal 803, a w-phase terminal 804and a neutral point terminal 805.

Then, the number of terminal of the transformer can be reduced from nineterminals to seven terminals in comparison with the first and secondembodiments.

FIG. 9 shows a polarity of a magnetomotive force that is generated ineach iron core by each winding of the transformer 801, and a wireconnection of the each winding. The transformer 801 includes iron cores202 to 204, and these iron cores 202 to 204 configure a three-leggedcore. A primary winding 200 has the same configuration with that of thefirst embodiment (FIG. 2).

A secondary winding 900 includes a u-phase winding 901, a v-phasewinding 902 and a w-phase winding 903. The number of winding of each ofthe windings 901 to 903 is substantially the same. The u-phase winding901, the v-phase winding 902 and the w-phase winding 903 arestar-connected, and a neutral point n is drawn from the neutral pointterminal 805.

The converter group 806 consists of a u-phase converter arm 807, av-phase converter arm 808 and a w-phase converter arm 809.

The a-terminal of the u-phase converter arm 807 is connected to apositive output terminal 121, and the b-terminal is connected to theu-phase terminal 802 of the transformer 801. In addition, in the thirdembodiment, an arm voltage of the u-phase converter arm 807 is denotedby Varmu.

The a-terminal of the v-phase converter arm 808 is connected to thepositive output terminal 121, and the b-terminal is connected to thev-phase terminal 803 of the transformer 801. In addition, in the thirdembodiment, an arm voltage of the v-phase converter arm 808 is denotedby Varmv.

The a-terminal of the w-phase converter arm 809 is connected to thepositive output terminal 121, and the b-terminal is connected to thew-phase terminal 804 of the transformer 801. In addition, in the thirdembodiment, an arm voltage of the w-phase converter arm 809 is denotedby Varmw.

The converter arms 807 to 809 in the third embodiment (FIG. 8) aresubstantially identical to the converter arms 113 to 115 and 117 to 119in FIG. 1 of the first embodiment and in FIG. 6 of the secondembodiment. However, the number of unit converter 120 included thereinis almost twice.

Fourth Embodiment

An explanation will be given of a fourth embodiment of the presentinvention. The fourth embodiment is a modification of the secondembodiment. In the second embodiment, two converter groups that are onthe positive side and the negative side are used. However, in the fourthembodiment, only one converter group is used, and effects identical tothose of the third embodiment can be obtained. In addition, in the thirdembodiment, the primary winding of the transformer is delta-connected.However, in the fourth embodiment, the primary winding of thetransformer is star-connected.

Below, an explanation will be given of only a part of configuration ofthe fourth embodiment different from the third embodiment.

FIG. 10 is a circuit diagram showing a fourth embodiment of the presentinvention. A power conversion device 1000 is interconnected with thethree-phase power system 100 through the three-phase AC terminals 102 to104, and transmits and receives active/reactive power to and from thethree-phase power system 100. The power conversion device 1000 consistsof a transformer 1001 and a converter group 805.

The transformer 1001 includes seven terminals in total that are theR-phase terminal 102, the S-phase terminal 103, the T-phase terminal104, the u-phase terminal 802, the v-phase terminal 803, the w-phaseterminal 804 and a neutral point terminal 805.

FIG. 11 shows a polarity of a magnetomotive force that is generated ineach iron core by each winding of the transformer 1001, and a wireconnection of the each winding. The transformer 1001 includes iron cores202 to 204, and these iron cores 202 to 204 configure a three-leggedcore. A primary winding 700 of FIG. 11 is identical to the primarywinding 700 in FIG. 7 of the second embodiment.

In addition, a secondary winding 900 is identical to the secondarywinding 900 in FIG. 9 of the third embodiment, and a converter group 706of FIG. 9 is identical to the converter group 706 in FIG. 7 of thesecond embodiment.

Fifth Embodiment

An explanation will be given of a fifth embodiment of the presentinvention. The fifth embodiment is a modification of the firstembodiment. Each phase of the secondary winding of the transformer isdivided into two, and a wire is connected so that a magnetomotive forceto be caused by a zero-phase current becomes zero.

In the fifth embodiment, effects identical to those of the firstembodiment can be obtained. In addition, when the current ID is appliedto the load device 123, a cross section of the iron core of thetransformer can be reduced in comparison with the embodiments 1 to 4.This is because, as described above, the magnetomotive force to becaused by the zero-phase current is zero.

FIG. 12 is a circuit diagram showing the fifth embodiment of the presentinvention. A configuration of FIG. 12 of the fifth embodiment has acircuit configuration that replaces the transformer 105 of FIG. 1 of thefirst embodiment with a transformer 1201.

The transformer 1201 includes nine terminals in total that are theR-phase terminal 102, the S-phase terminal 103, the T-phase terminal104, a u-phase positive terminal 1202, a v-phase positive terminal 1203,a w-phase positive terminal 1204, a u-phase negative terminal 1206, av-phase negative terminal 1207 and a w-phase negative terminal 1208.

FIG. 13 shows a polarity of a magnetomotive force that is generated ineach iron core by each winding of the transformer 1201, and a wireconnection of the each winding. The transformer 1201 includes iron cores202 to 204, and these iron cores 202 to 204 configure a three-leggedcore. A primary winding 200 of FIG. 13 is identical to the primarywinding 200 of FIG. 1 of the first embodiment.

A secondary winding 1300 includes a u-phase positive winding 1301, av-phase positive winding 1302, a w-phase positive winding 1303, au-phase negative winding 1304, a v-phase negative winding 1305 and aw-phase negative winding 1306. The number of winding of each of thewindings 1301 to 1306 is substantially the same.

The u-phase positive winding 1301 and the u-phase negative winding 1304are electrically connected in series. The u-phase positive winding 1301is wound on the iron core 202, and the u-phase negative winding 1304 iswound on the iron core 204. Meanwhile, the wire is connected so that amagnetomotive force to be generated in the iron core 202 by the u-phasepositive winding 1301 and the magnetomotive force to be generated in theiron core 204 by the u-phase negative winding 1304 have substantiallythe same strength with reverse polarity to each other.

The v-phase positive winding 1302 and the v-phase negative winding 1305are electrically connected in series. The v-phase positive winding 1302is wound on the iron core 203, and the v-phase negative winding 1305 iswound on the iron core 202. Meanwhile, the wire is connected so that amagnetomotive force to be generated in the iron core 203 by the v-phasepositive winding 1302 and the magnetomotive force to be generated in theiron core 202 by the v-phase negative winding 1305 have substantiallythe same strength with reverse polarity to each other.

The w-phase positive winding 1303 and the w-phase negative winding 1306are electrically connected in series. The w-phase positive winding 1303is wound on the iron core 204, and the w-phase negative winding 1306 iswound on the iron core 203. Meanwhile, the wire is connected so that amagnetomotive force to be generated in the iron core 204 by the w-phasepositive winding 1303 and the magnetomotive force to be generated in theiron core 203 by the w-phase negative winding 1306 have substantiallythe same strength with reverse polarity to each other.

In the embodiment, the u-phase positive winding 1301 and the u-phasenegative winding 1304 are generally called a u-phase winding. Inaddition, the v-phase positive winding 1302 and the v-phase negativewinding 1305 are generally called a v-phase winding. Similarly, thew-phase positive winding 1303 and the w-phase negative winding 1306 aregenerally called a w-phase winding.

In the embodiment, a voltage between both ends of the u-phase positivewinding 1301 is denoted by VuH, a voltage between both ends of thev-phase positive winding 1302 is denoted by VvH, a voltage between bothends of the w-phase positive winding 1303 is denoted by VwH, a voltagebetween both ends of the u-phase negative winding 1304 is denoted byVuL, a voltage between both ends of the v-phase negative winding 1305 isdenoted by VvL and a voltage between both ends of the w-phase negativewinding 1306 is denoted by VwL.

In addition, a sum of VuH and VuL is called a u-phase voltage Vu, a sumof VvH and VvL is called a v-phase voltage Vv and a sum of VwH and VwLis called a w-phase voltage Vw.

In FIG. 14, phasor diagrams of voltages (that is, line voltages VRS, VSTand VTR of the three-phase power system 100) of the primary winding 200and voltages VuH, VvH, VwH, VuL, VvL, VwL, Vu, Vv and Vw of thesecondary winding 1300 of the transformer 1201 are shown.

A positive converter group 112 of FIG. 12 is identical to the convertergroup 112 in FIG. 1 of the first embodiment. In addition, a negativeconverter group 116 of FIG. 12 is identical to the converter group 116in FIG. 1 of the first embodiment.

An explanation will be given below of differences between the firstembodiment and the fifth embodiment. As described in the firstembodiment, when the power conversion device 101 transmits and receivesactive power to and from the three-phase power system 100, a current IDflows. In addition, when power conversion device 101 supplies asingle-phase reactive power to the load device 123, the current ID alsoflows. The current ID dividedly flows into each converter arm ofrespective phases substantially equally to form a zero-phase component(zero-phase current) of each of the arm currents Iu, Iv and Iw. Then,when the current ID flows, the zero-phase current flows in the secondarywinding 201. In the embodiment, the zero-phase current is denoted by Iz.

When the current ID is a DC current, there is a possibility that azero-phase DC current flows in the secondary winding and causes a directcurrent bias magnetism and magnetic saturation.

Meanwhile, a case that the power conversion device 1200 of the fifthembodiment supplies the current ID to the load device 123 is considered.As with the case of the first embodiment, a zero-phase current flows inthe secondary winding 1300 of the transformer 1201.

In the iron core 202, a magnetomotive force generated by Iz flowing inthe u-phase positive winding 1301 and the magnetomotive force generatedby Iz flowing in the v-phase negative winding 1305 have substantiallythe same strength with reverse polarity to each other, thereby resultingin substantially canceling the magnetomotive force.

In the iron core 203, a magnetomotive force generated by Iz flowing inthe v-phase positive winding 1302 and the magnetomotive force generatedby Iz flowing in the w-phase negative winding 1306 have substantiallythe same strength with reverse polarity to each other, thereby resultingin substantially canceling the magnetomotive force.

In the iron core 204, a magnetomotive force generated by Iz flowing inthe w-phase positive winding 1303 and the magnetomotive force generatedby Iz flowing in the u-phase negative winding 1304 have substantiallythe same strength with reverse polarity to each other, thereby resultingin substantially canceling the magnetomotive force.

Therefore, even if the ID is a DC current, a direct currentmagnetomotive force becomes substantially zero. Therefore, a directcurrent bias magnetism of the iron core is hardly generated.

Below, an explanation will be given of operations of the powerconversion device 1200, using FIG. 15. FIG. 15 is examples of operationwaveforms of the power conversion device 1200, and demonstrates briefwaveforms of line voltages VRS, VST and VTR of the three-phase powersystem 100, system currents IR, IS and IT, arm voltages Varmu, Varmv andVarmw, zero-phase component (Varmu+Varmv+Varmw)/3 of arm voltage, armcurrents Iu, Iv and Iw, and an output terminal current ID.

In FIG. 15, the power conversion device 1200 receives active power fromthe system with power factor 1 and applies a DC current to the loaddevice 123 so that a DC current flows therein. That is, the VD and theID are both direct current.

In the explanation of FIG. 15, the unit converter 120 is a bidirectionalchopper circuit shown in FIG. 4.

The arm voltages Varmu, Varmv and Varmw of respective converter arms area multilevel waveform having the number of levels substantially equal tothe number of unit converter 120 included in the respective converterarms. In addition, the arm voltages Varmu, Varmv and Varmw contain athree-phase AC component and a zero-phase DC component. The zero-phaseDC component (Varmu+Varmv+Varmw)/3 of the arm voltages Varmu, Varmv andVarmw is substantially equal to the output terminal voltage VD.

Since Varmu, Varmv and Varmw include a zero-phase DC componentsubstantially equal to the VD, the u-phase voltage Vu, the v-phasevoltage Vv and the w-phase voltage Vw have voltages with reverse-phasesof AC components of Varmu, Varmv and Varmw, respectively, according toNumber 1, Number 2 and Number 3, and their DC components are almostnothing.

Voltage differences between Vu, Vv and Vw and voltages aVRS, aVST andaVTR, which are voltages of the line voltages VRS, VST and VTR of thethree-phase power system 100 converted to the secondary side of thetransformer 1201, are supported by a leakage inductance of thetransformer 1201 between the primary winding 200 and the secondarywinding 1300.

If only phases of Vu, Vv and Vw are controlled to be slightly delayed incomparison with the phases of aVRS, aVST and aVTR, while controllingfrequencies and amplitudes of Vu, Vv and Vw to be identical to those ofaVRS, aVST and aVTR, an active current flows into the power conversiondevice 1200 from the three-phase power system 100.

A voltage difference between a zero-phase component of Varmu, Varmv andVarmw and the voltage VD of the output terminal is supported by aninductance for the zero-phase component of the secondary winding 1300. Azero-phase component Iz of Iu, Iv and Iw is proportional to timeintegration of the voltage difference. Therefore, the zero-phasecomponent Iz of Iu, Iv and Iw can be controlled by controlling azero-phase DC component of Varmu, Varmv and Varmw. A sum of zero-phasecomponent of Iu, Iv and Iw is the ID.

FIG. 16 is an example of a DC transmission system to which the powerconversion device 1200 based on the present embodiment is applied. Thepower conversion device 1200 on land A is interconnected with athree-phase power system 1600. The power conversion device 1200 on landB is interconnected with a three-phase power system 1601. Outputterminals 120 and 121 of the two power conversion devices 1200 areconnected by a submarine cable, and electric power is transmitted backand forth between the land A and the land B.

Sixth Embodiment

FIG. 17 is a circuit diagram showing a sixth embodiment of the presentinvention. In the foregoing fifth embodiment, the primary winding of thetransformer is delta-connected. However, in the sixth embodiment, theprimary winding of a transformer is star-connected and obtains effectsidentical to those of the fifth embodiment.

Below, an explanation will be given of only a part of the configurationof the sixth embodiment different from the fifth embodiment.

A positive converter group 112 of FIG. 17 is identical to the positiveconverter group 112 of the first embodiment (FIG. 1). In addition, anegative converter group 116 of FIG. 17 is identical to the negativeconverter group 116 of the first embodiment (FIG. 1).

FIG. 18 shows a polarity of a magnetomotive force that is generated ineach iron core by each winding of the transformer 1701, and a wireconnection of the each winding. The transformer 1701 includes iron cores202 to 204, and these iron cores 202 to 204 configure a three-leggedcore. A primary winding 700 of FIG. 18 is identical to the primarywinding 700 of FIG. 7 of the second embodiment. In addition, a secondarywinding 1300 of FIG. 18 is identical to the secondary winding 1300 ofFIG. 13 of the fifth embodiment.

Seventh Embodiment

FIG. 19 is a circuit diagram showing a seventh embodiment of the presentinvention. The seventh embodiment is a modification of the fifthembodiment. In the fifth embodiment, two converter groups on thepositive side and the negative side are used. However, in the seventhembodiment, only one converter group is used.

In the seventh embodiment, effects identical to those of the fifthembodiment can be obtained, and the number of terminals of thetransformer can be reduced from nine terminals to five terminals as withthe third embodiment.

Below, an explanation will be given of only a part of configuration ofthe seventh embodiment different from the fifth embodiment.

A converter group 806 of FIG. 19 is identical to the converter group 806of FIG. 8 of the third embodiment.

The transformer 1901 includes seven terminals in total that are theR-phase terminal 102, the S-phase terminal 103, the T-phase terminal104, an u-phase terminal 1902, a v-phase terminal 1903, a w-phaseterminal 1904 and a neutral point terminal 1905.

FIG. 20 shows a polarity of a magnetomotive force that is generated ineach iron core by each winding of the transformer 1901, and a wireconnection of the each winding. The transformer 1901 includes iron cores202 to 204, and these iron cores 202 to 204 configure a three-leggedcore. A primary winding 200 of FIG. 20 is identical to the primarywinding 200 of FIG. 2 of the first embodiment.

A u-phase positive winding 1301, a v-phase positive winding 1302, aw-phase positive winding 1303, a u-phase negative winding 1304, av-phase negative winding 1305 and a w-phase negative winding 1306 thatconfigure secondary winding 2000 of FIG. 20 are identical to the u-phasepositive winding 1301, the v-phase positive winding 1302, the w-phasepositive winding 1303, the u-phase negative winding 1304, the v-phasenegative winding 1305 and the w-phase negative winding 1306 of FIG. 13of the fifth embodiment.

However, the u-phase negative winding 1304, the v-phase negative winding1305 and the w-phase negative winding 1306 are star-connected, and aneutral point n is drawn out outside the transformer 1901 as a neutralpoint terminal 1905.

Eighth Embodiment

An explanation will be given of an eighth embodiment embodying thepresent invention. The eighth embodiment is a modification of theseventh embodiment. In the seventh embodiment, the primary winding ofthe transformer is delta-connected. However, in the eighth embodiment,the primary winding is star-connected, and effects identical to those ofthe seventh embodiment are obtained.

Below, an explanation will be given of only a part of configuration ofthe eighth embodiment different from the seventh embodiment.

FIG. 21 is a circuit diagram showing an eighth embodiment of the presentinvention.

A converter group 806 of FIG. 21 is identical to the converter group 806of FIG. 8 of the third embodiment.

The transformer 2100 includes seven terminals in total that are theR-phase terminal 102, the S-phase terminal 103, the T-phase terminal104, an u-phase terminal 1902, a v-phase terminal 1903, a w-phaseterminal 1904 and a neutral point terminal 1905.

FIG. 22 shows a polarity of a magnetomotive force that is generated ineach iron core by each winding of the transformer 2101, and a wireconnection of the each winding. The transformer 2101 includes iron cores202 to 204, and these iron cores 202 to 204 configure a three-leggedcore. A primary winding 700 of FIG. 22 is identical to the primarywinding 700 of FIG. 7 of the second embodiment. A secondary winding 2000of FIG. 22 is identical to the secondary winding 2000 of FIG. 20 of theseventh embodiment.

Ninth Embodiment

An explanation will be given of a ninth embodiment embodying the presentinvention. In the fifth to eighth embodiments, the secondary winding ofthe transformer for each phase is divided into two, and connected sothat a magnetomotive force to be caused by the zero-phase currentbecomes zero. On the other hand, in the ninth embodiment, the effectsidentical to those of the seventh embodiment can be obtained by using acompensating winding which compensates the magnetomotive force to becased by the zero-phase current.

Below, an explanation will be given of only a part of configuration ofthe ninth embodiment different from the seventh embodiment.

FIG. 23 is a circuit diagram showing a ninth embodiment of the presentinvention. A power conversion device 2300 is interconnected with thethree-phase power system 100 through the three-phase AC terminals 102 to104, and transmits and receives active/reactive power to and from thethree-phase power system 100. The power conversion device 2300 consistsof a transformer 2301 and a converter group 806.

The converter group 806 of FIG. 23 is identical to the converter group806 of FIG. 8 of the third embodiment.

A transformer 2301 includes seven terminals in total that are theR-phase terminal 102, the S-phase terminal 103, the T-phase terminal104, an u-phase terminal 2302, a v-phase terminal 2303, a w-phaseterminal 2304 and a compensating winding terminal 2305.

FIG. 24 shows a polarity of a magnetomotive force that is generated ineach iron core by each winding of the transformer 2301, and a wireconnection of the each winding. The transformer 2301 includes iron cores202 to 204, a primary winding 200, a secondary winding 2400 and acompensating winding 2404. The iron cores 202 to 204 configure athree-legged core.

The primary winding 200 is identical to the primary winding 200 of FIG.1 of the first embodiment.

The secondary winding 2400 includes a u-phase winding 2401, a v-phasewinding 2402 and a w-phase winding 2403. The secondary winding 2400 hassubstantially the same configuration with the secondary winding 900 inFIG. 9 of the third embodiment except that the neutral point n isconnected to the compensating winding 2404.

The compensating winding 2404 includes a u-phase compensating winding2405, a v-phase compensating winding 2406 and a w-phase compensatingwinding 2407. The number of winding of the compensating winding 2404 isset to ⅓ of that of the secondary winding 2400.

The u-phase compensating winding 2405 is wound on the iron core 202. Thev-phase compensating winding 2406 is wound on the iron core 203. Inaddition, the w-phase compensating winding 2407 is wound on the ironcore 204.

The u-phase compensating winding 2405, the v-phase compensating winding2406 and the w-phase compensating winding 2407 are connected in series.

One end of the compensating winding 2404 is connected to the negativeoutput terminal 122. Therefore, a current ID flowing in the load device123 flows in the compensating winding 2404.

A magnetomotive force that is generated in the iron core 202 by thecurrent ID flowing in the u-phase compensating winding 2405, amagnetomotive force that is generated in the iron core 203 by thecurrent ID flowing in the v-phase compensating winding 2406 and amagnetomotive force that is generated in the iron core 204 by thecurrent ID flowing in the w-phase compensating winding 2407 havesubstantially the same strength of same polarity.

The current ID flowing in the compensating winding 2404 is branched atthe neutral point n, and flows as a zero-phase component Iz of thesecondary winding 2400. That is, Iz=ID/3.

A magnetomotive force that is generated in the iron core 202 by Izflowing in the u-phase winding 2401 and a magnetomotive force that isgenerated in the iron core 202 by Iz flowing in the u-phase compensatingwinding 2405 have substantially the same strength with reverse polarityto each other, thereby resulting in cancellation of the magnetomotiveforce.

A magnetomotive force that is generated in the iron core 203 by Izflowing in the v-phase winding 2402 and a magnetomotive force that isgenerated in the iron core 203 by Iz flowing in the v-phase compensatingwinding 2406 have substantially the same strength with reverse polarityto each other, thereby resulting in cancellation of the magnetomotiveforce.

A magnetomotive force that is generated in the iron core 204 by Izflowing in the w-phase winding 2403 and a magnetomotive force that isgenerated in the iron core 204 by Iz flowing in the w-phase compensatingwinding 2407 have substantially the same strength with reverse polarityto each other, thereby resulting in cancellation of the magnetomotiveforce.

Therefore, as with the fifth to eighth embodiments, even if the currentID is a DC current, a direct current magnetomotive force becomessubstantially zero. Then, a direct current bias magnetism of the ironcore is hardly generated.

Tenth Embodiment

An explanation will be given of a tenth embodiment embodying the presentinvention. In the ninth embodiment, the primary winding of thetransformer is delta-connected. However, in the tenth embodiment, theprimary winding is star-connected, and the effects identical to those ofthe ninth embodiment are obtained.

Below, an explanation will be given of only a part of configuration ofthe tenth embodiment different from the ninth embodiment.

FIG. 25 is a circuit diagram showing the tenth embodiment of the presentinvention. The power conversion device 2500 is interconnected with thethree-phase power system 100 through the three-phase AC terminals 102 to104, and transmits and receives active/reactive power to and from thethree-phase power system 100. The power conversion device 2500 consistsof a transformer 2501 and a converter group 806.

A converter group 806 of FIG. 25 is identical to the converter group 806of FIG. 8 of the third embodiment.

FIG. 26 shows a polarity of a magnetomotive force that is generated ineach iron core by each winding of the transformer 2501, and a wireconnection of the each winding. The transformer 2501 includes iron cores202 to 204, a primary winding 700, a secondary winding 2400 and acompensating winding 2404. The iron cores 202 to 204 configure athree-legged core.

The primary winding 700 of FIG. 26 is identical to the primary winding700 of FIG. 7 of the second embodiment.

The secondary winding 2400 of FIG. 26 is identical to the secondarywinding 2400 of FIG. 24 of the ninth embodiment.

Eleventh Embodiment

An explanation will be given of an eleventh embodiment embodying thepresent invention. The eleventh embodiment is a modification of thefifth embodiment, and in the eleventh embodiment, the effects identicalto those of the fifth embodiment can be obtained.

Below, an explanation will be given of only a part of configuration ofthe eleventh embodiment different from the fifth embodiment.

FIG. 27 is a circuit diagram showing the eleventh embodiment of thepresent invention. In comparison with FIG. 12 of the fifth embodiment,polarities of the u-phase negative converter arm 117, v-phase negativeconverter arm 118 and the w-phase negative converter arm 118 of FIG. 12of the fifth embodiment are reversed in the eleventh embodiment.

Similarly, in the first, second, fifth and sixth embodiments, thepolarities of the u-phase negative converter arm 117, v-phase negativeconverter arm 118 and the w-phase negative converter arm 118 may bereversed.

Twelfth Embodiment

Other embodiments of the present invention will be explained below,using drawings.

An explanation will be given of a twelfth embodiment embodying thepresent invention.

FIG. 28 is a circuit diagram of a DC transmission system using a powerconversion device of the present invention.

First, a configuration of the DC transmission system of the presentinvention will be explained. The DC transmission system includesthree-phase AC power systems 3100, 3170 and two sets of two powerconversion devices 3101, each set being interconnected with each of thethree-phase AC power systems 3100, 3170, and one of the two DC outputterminals of each of the two power conversion devices 3101 that areinterconnected with each of the three-phase AC power systems 3100, 3170is connected to a DC transmission cable 3150 and the other of the two DCoutput terminals is grounded.

The DC transmission system of the present invention converts AC powerfrom the three-phase AC power systems 3100 and 3170 to DC power usingthe two power conversion devices 3101 interconnected with eachthree-phase AC power system, and transmits electric power in onedirection or both directions through the DC transmission cable 3150.

Next, an explanation will be given of a configuration of the powerconversion device 3101. The power conversion device 3101 includes atransformer 3105, a positive converter group 3112 and a negativeconverter group 3116.

In the specification, each phase of the three-phase AC power system 3100is called an R-phase, an S-phase and a T-phase. In addition, a currentflowing in each phase of the three-phase AC power system 3100 is calleda system current and denoted by IR, IS and IT.

Next, an explanation will be given of a configuration of the transformer3105, using FIG. 28 and FIG. 29.

The transformer 3105 includes nine terminals in total that are anR-phase terminal 3102, an S-phase terminal 3103, a T-phase terminal3104, an u-phase positive terminal 3106, a v-phase positive terminal3107, a w-phase positive terminal 3108, an u-phase negative terminal3109, a v-phase negative terminal 3110 and a w-phase negative terminal3111.

FIG. 29 shows a polarity of a magnetomotive force that is generated ineach iron core by each winding of the transformer 3105, and a wireconnection of the each winding. The transformer 3105 includes iron cores3131 to 3133, and these iron cores 3131 to 3133 configure a three-leggedcore. The primary winding is delta-connected, and windings 3202 to 3204between the R-phase and the S-phase, the S-phase and the T-phase, andthe T-phase and the R-phase, respectively are wound on respective ironcores 3131 to 3133. The number of winding of each of the windings 3202to 3204 is substantially the same.

The u-phase positive winding 3134 and the u-phase negative winding 3137are electrically connected in series. The u-phase positive winding 3134is wound on the iron core 3131, and the u-phase negative winding 3137 iswound on the iron core 3133. Meanwhile, the wire is connected so that amagnetomotive force to be generated in the iron core 3131 by the u-phasepositive winding 3134 and the magnetomotive force to be generated in theiron core 3133 by the u-phase negative winding 3137 have substantiallythe same strength with reverse polarity to each other.

The v-phase positive winding 3135 and the v-phase negative winding 3138are electrically connected in series. The v-phase positive winding 3135is wound on the iron core 3132, and the v-phase negative winding 3138 iswound on the iron core 3131. Meanwhile, the wire is connected so that amagnetomotive force to be generated in the iron core 3132 by the v-phasepositive winding 3135 and the magnetomotive force to be generated in theiron core 3131 by the v-phase negative winding 3138 have substantiallythe same strength with reverse polarity to each other.

The w-phase positive winding 3136 and the w-phase negative winding 3139are electrically connected in series. The w-phase positive winding 3136is wound on the iron core 3133, and the w-phase negative winding 3139 iswound on the iron core 3132. Meanwhile, the wire is connected so that amagnetomotive force to be generated in the iron core 3133 by the w-phasepositive winding 3136 and the magnetomotive force to be generated in theiron core 3132 by the w-phase negative winding 3139 have substantiallythe same strength with reverse polarity to each other.

In the specification, the u-phase positive winding 3134 and the u-phasenegative winding 3137 are generally called a u-phase winding. Inaddition, the v-phase positive winding 3135 and the v-phase negativewinding 3138 are generally called a v-phase winding. Similarly, thew-phase positive winding 3136 and the w-phase negative winding 3139 aregenerally called a w-phase winding.

In the specification, a voltage between both ends of the u-phasepositive winding 3134 is described by VuH, a voltage between both endsof the v-phase positive winding 3135 is denoted by VvH, a voltagebetween both ends of the w-phase positive winding 3136 is denoted byVwH, a voltage between both ends of the u-phase negative winding 3137 isdenoted by VuL, a voltage between both ends of the v-phase negativewinding 3138 is denoted by VvL and a voltage between both ends of thew-phase negative winding 3139 is denoted by VwL.

In addition, a sum of VuH and VuL is called a u-phase voltage Vu, a sumof VvH and VvL is called a v-phase voltage Vv and a sum of VwH and VwLis called a w-phase voltage Vw.

In addition, a voltage applied between a positive DC output terminal3121 and a negative DC output terminal 3122 of the power conversiondevice 3101 is denoted by VD, and a current flowing in the positive DCoutput terminal 3121 is denoted by ID.

Next, explanations will be given of configurations of the positiveconverter group 3112 and the negative converter group 3116.

The positive converter group 3112 consists of a u-phase positiveconverter arm 3113, a v-phase positive converter arm 3114 and a w-phasepositive converter arm 3115. In addition, the negative converter group3116 consists of a u-phase negative converter arm 3117, a v-phasenegative converter arm 3118 and a w-phase negative converter arm 3119.

Each of the converter arms 3113 to 3115 and 3117 to 3119 has aa-terminal and a b-terminal.

In the specification, a voltage of the a-terminal against a standardvoltage of the b-terminal is called an arm voltage. In addition, theeach of the converter arms 3113 to 3115 and 3117 to 3119 is a circuitthat cascade-connects one or a plurality of unit chopper cells shown inFIG. 30.

The a-terminal of the u-phase positive converter arm 3113 is connectedto the positive output terminal 3121, and the b-terminal is connected tothe u-phase positive terminal 3106 of the transformer 3105. In addition,in the specification, the arm voltage of the u-phase positive converterarm 3113 is denoted by VarmuH.

The a-terminal of the v-phase positive converter arm 3114 is connectedto the positive output terminal 3121, and the b-terminal is connected tothe v-phase positive terminal 3107 of the transformer 3105. In addition,in the specification, the arm voltage of the v-phase positive converterarm 3114 is denoted by VarmvH.

The a-terminal of the w-phase positive converter arm 3115 is connectedto the positive output terminal 3121, and the b-terminal is connected tothe w-phase positive terminal 3108 of the transformer 3105. In addition,in the specification, the arm voltage of the w-phase positive converterarm 3115 is denoted by VarmwH.

The a-terminal of the u-phase negative converter arm 3117 is connectedto the u-phase negative terminal 3109, and the b-terminal is connectedto the negative output terminal 3122. In addition, in the specification,the arm voltage of the u-phase negative converter arm 3117 is denoted byVarmuL.

The a-terminal of the v-phase negative converter arm 3118 is connectedto the v-phase negative terminal 3110, and the b-terminal is connectedto the negative output terminal 3122. In addition, in the specification,the arm voltage of the v-phase negative converter arm 3118 is denoted byVarmvL.

The a-terminal of the w-phase negative converter arm 3119 is connectedto the w-phase negative terminal 3111, and the b-terminal is connectedto the negative output terminal 3122. In addition, in the specification,the arm voltage of the w-phase negative converter arm 3119 is denoted byVarmwL.

In the twelfth embodiment, a sum of VarmuH and VarmuL is denoted byu-phase arm voltage Varmu. In addition, a sum of VarmvH and VarmvL isdenoted by v-phase arm voltage Varmv. Similarly, a sum of VarmwH andVarmwL is denoted by w-phase arm voltage Varmw.

In addition, in the twelfth embodiment, a current flowing in the u-phasepositive converter arm 3113 and u-phase negative converter arm 3117 isdenoted by u-phase arm current Iu, a current flowing in the v-phasepositive converter arm 3114 and v-phase negative converter arm 3118 isdenoted by v-phase arm current Iv, and a current flowing in the w-phasepositive converter arm 3115 and w-phase negative converter arm 3119 isdenoted by w-phase arm current Iw.

Next, an explanation will be given of a configuration of the unitchopper cell 3120, using FIG. 30.

A unit chopper cell shown in FIG. 30 consists of a high-side switchingdevice 3303, a low-side switching device 3304 and an energy storagedevice 3305. The switching devices 3303, 3304 are semiconductorswitching devices represented by IGBT. In addition, the energy storagedevice 3305 is, for example, a capacitor or a battery. In thespecification, a voltage of x-terminal 3301 against a standard voltageof y-terminal 3302 is denoted by a cell voltage Vcell of the unitchopper cell.

Next, operations of the power conversion devices 3101 will be explainedfor the following two cases.

(1) A case that the power conversion device 3101 receives active powerfrom the three-phase system 100 and supplies DC power to the DCtransmission cable 3150.(2) A case that the power conversion device 3101 receives DC power fromthe DC transmission cable 3150 and supplies active power to thethree-phase system 100.

Below, an explanation will be given of operations in the case that thepower conversion device 3101 receives active power from the three-phasesystem 100 and supplies a DC power to the DC transmission cable 3150.

In the specification, voltages of the line voltages VRS, VST and VTR ofthe three-phase power system 100 converted to the voltages on thesecondary side of the transformer are denoted by aVRS, aVST and aVTR.Here, a is a turn ratio of the secondary winding to the primary windingof the transformer.

Here, an explanation will be given of a relationship among voltages Vu,Vv, Vw of secondary winding of a transformer, arm voltages Varmu, Varmv,Varmw and a voltage VD to be applied between the positive DC outputterminal 3121 and the negative DC output terminal 3122.

A relationship among Vu, Varmu and VD is expressed by the followingformula.

Vu=VD−Varmu  [Number 1]

A relationship among Vv, Varmv and VD is expressed by the followingformula.

Vv=VD−Varmv  [Number 2]

A relationship among Vw, Varmw and VD is expressed by the followingformula.

Vw=VD−Varmw  [Number 3]

According to the Numbers 1 to 3, the voltages Vu, Vv, Vw of thesecondary winding of the transformer can be controlled by controllingthe u-phase arm voltage Varmu, the v-phase arm voltage Varmv and thew-phase arm voltage Varmw.

If only phases of Vu, Vv and Vw are slightly delayed in comparison withthe phases of aVRS, aVST and aVTR, while matching frequencies andamplitudes of Vu, Vv and Vw with those of aVRS, aVST and aVTR, an activecurrent flows into the power conversion device 3101 from the three-phaseAC power system 100.

Next, an explanation will be given about that the arm voltage can becontrolled by a switching condition of a semiconductor switching deviceconstituting the unit chopper cell 3120.

When the high-side switching device 3303 is ON and low-side switchingdevice 3304 is OFF, the cell voltage Vcell is substantially equal to thevoltage VC of the DC capacitor 3305 without depending on the currentIcell.

When the high-side switching device 3303 is OFF and low-side switchingdevice 3304 is ON, the cell voltage Vcell is substantially zero withoutdepending on the current Icell.

When the high-side switching device 3303 and low-side switching device3304 are both OFF, the cell voltage Vcell is determined depending on apolarity of the current Icell. If the Icell is positive, the cellvoltage Vcell is substantially equal to the voltage VC of the energystorage device 3305. If the Icell is negative, the cell voltage Vcell issubstantially zero.

Next, an explanation will be given of a method for supplying electricpower to the DC transmission cable 3150.

The current ID flowing in the DC transmission cable 3150 is a sum(Iu+Iv+Iw) of Iu, Iv and Iw. When arm voltages Varmu, Varmv and Varmw donot contain a zero-phase component, arm currents Iu, Iv and Iw also donot contain a zero-phase component. When the arm currents Iu, Iv and Iwdo not contain the zero-phase component, it becomes that Iu+Iv+Iw=ID=0,and electric power can not be transmitted to the DC transmission cable3150.

In this case, active power flowed into the power conversion device 3101from the three-phase AC power system 3100 is stored in the energystorage device (for example, electrolytic capacitor) in each of the unitconverters 3120.

In order to supply electric power to the DC transmission cable, thezero-phase component of the arm voltages Varmu, Varmv and Varmw isadjusted and the zero-phase component of the arm currents Iu, Iv and Iwis controlled. According to the Kirchhoff's current law, since itbecomes that ID=Iu+Iv+Iw, the current ID can be supplied by adjustingthe zero-phase component of the arm currents Iu, Iv and Iw.

Meanwhile, when active power flowing into the power conversion device3101 from the three-phase AC power system 3100 is equal to the activepower transmitted to the DC transmission cable 3150, an amount of energyflowing into/out from each unit chopper cell 3120 during one cycle ofthree-phase AC power system is substantially zero.

In addition, as the current ID, a direct current, an alternate current,or a current that alternate current is superimposed on direct currentmay be used.

Below, an explanation will be given of operations in the case that thepower conversion device 3101 receives active power from the DCtransmission cable 3150 and supplies active power to the three-phase ACpower system 3100.

If only phases of Vu, Vv and Vw are slightly advanced in comparison withthe phases of aVRS, aVST and aVTR, while matching frequencies andamplitudes of Vu, Vv and Vw with those of aVRS, aVST and aVTR, activepower can be supplied to the three-phase power system 3100 from thepower conversion device 3101.

Next, an explanation will be given of a method for receiving power fromthe DC transmission cable 3150.

The current ID flowing out from a DC transmission cable is a sum(Iu+Iv+Iw) of arm currents Iu, Iv and Iw. When the arm voltages Varmu,Varmv and Varmw do not contain a zero-phase component, the arm currentsIu, Iv and Iw also do not contain a zero-phase component. When the armcurrents Iu, Iv and Iw do not contain the zero-phase component, itbecomes that Iu+Iv+Iw=ID=0, and electric power can not be supplied fromthe DC transmission cable 3150.

In this case, active power flowing into the three-phase power system3100 from the power conversion device 3101 is supplied from the energystorage device (for example, electrolytic capacitor) in each unitchopper cell 3120.

In order to have electric power flow into the power conversion device3101 from the DC transmission cable 3150, the zero-phase component ofthe arm voltages Vsrmu, Varmv and Varmw is adjusted and the zero-phasecomponent of the arm currents Iu, Iv and Iw is controlled. According tothe Kirchhoff's current law, since it becomes that ID=Iu+Iv+Iw, thecurrent ID can be supplied by adjusting the zero-phase component of thearm currents Iu, Iv and Iw.

Meanwhile, if active power flowing into the three-phase power system3100 from the power conversion device 3101 is equal to the active powerflowing into the power conversion device 3101 from the DC transmissioncable 3150, an amount of energy flowing into/out from each unit choppercell 3120 during one cycle of three-phase power system 3100 issubstantially zero.

In the specification, a line between a positive DC output terminal 3121and a negative DC output terminal 3122 of each of the two powerconversion devices 3101, which are interconnected with the three-phaseAC power systems 3100 and 3170, including the DC transmission cable 3150is called a DC line.

In addition, in the specification, a line between the transformer 3105including AC output terminals 3102 to 3104 for respective phases and thethree-phase AC power systems 3100, 3170 is called an AC line.

Next, an explanation will be given about that an operation of the powerconversion device 3101 is different between short-circuiting of the ACline and the short-circuiting of the DC line.

When the AC line is short-circuited, if the power conversion device 3101is outputting a voltage to the AC line, a short-circuit current flows.In order to prevent the short-circuit current, as with a general powerconversion device, the high-side switching device 3303 and the low-sideswitching device 3304 which constitute each unit chopper cell 3120 areboth turned OFF, and the AC line is prevented from flowing of anovercurrent.

When the DC line is short-circuited, charges stored in an energy storagedevice 3305 in each unit chopper cell 3120 are discharged to the DCline, and the current ID becomes an overcurrent. In order to prevent theovercurrent, in each unit chopper cell, the high-side switching device3303 is turned OFF and the low-side switching device 3304 is turned ON.A diode is connected to the high-side switching device 3303 inanti-parallel. Since the diode has a reverse blocking characteristic, aDC voltage of the energy storage device 3305 is electrically insulatedfrom the DC line. Therefore, the overcurrent into the DC transmissioncable 3150 can be suppressed.

Next, as described above, since the protection operation of the powerconversion device 3101 is different between the short-circuiting of ADline and the short-circuiting of DC line, it is required to distinguishthe short-circuiting of AD line from the short-circuiting of DC line.

When the AC line is short-circuited, a current detected by a currentcensor set in the primary winding side or the secondary winding side ofthe transformer 3105 increases. Therefore, if the current detected bythe current censor set in the primary winding side or the secondarywinding side of the transformer 3105 exceeds a predetermined thresholdvalue, it is determined that the AC line is in trouble.

In addition, current censors are set for respective phases of thepositive converter group 3112 and the negative converter group 3116, andif a difference between current values detected by the current censorset in the positive converter group 3112 and detected by the currentcensor set in the negative converter group 3116 exceeds a predeterminedthreshold value, it may be determined that the AC line is in trouble.

When the DC line is short-circuited, if a current detected by a currentcensor 3123 set in the DC line exceeds a predetermined threshold value,it is determined that the DC line is in trouble.

In addition, a current censor is set in the a-terminal or the b-terminalof the converter arm of each phase, and if a sum of currents of thethree phases flowing in the respective converter arms exceeds apredetermined threshold value, it may be determined that the DC line isin trouble.

If it is determined that the AC line or DC line is in trouble, the powerconversion device 3101 is disconnected from the three-phase AC powersystem 3100 or 3170 in a short time (generally, several tens ofmilliseconds to several hundreds of milliseconds) by a breaker 3124.

When the DC line is short-circuited, a short-circuit current Ish flowsfrom the three-phase AC power system 3100 or 3170 depending on a leakageimpedance of the transformer 3105 during a short time until the powerconversion device 3101 is disconnected from the three-phase AC powersystem 3100 or 3170 by the breaker 3124. If a voltage of the three-phaseAC power system 3100 or 3170 is denoted by Vs and the leakage impedanceof the transformer 3105 is denoted by Ztr, the short-circuit current Ishis expressed by the following formula.

Ish=Vs/Ztr  [Number 4]

When a saturation current of a semiconductor switching deviceconstituting the unit chopper cell 3120 is denoted by Isa, if theleakage impedance Ztr of the transformer 3105 is adjusted so as tosatisfy the condition of Isa>Ish, then, the semiconductor switchingdevice can be protected.

In addition, the semiconductor switching device can be protected bydisposing a cooling system of the semiconductor switching device so thata junction temperature of the semiconductor switching device does notexceed a predetermined temperature during a time (generally, severaltens of milliseconds to several hundreds of milliseconds) until thepower conversion device 3101 is disconnected from the three-phase ACpower system 3100 or 3170 by the breaker 3124.

Next, an explanation will be given of an example of a cooling featureusing FIG. 37.

FIG. 37 is an illustration showing one example of the cooling feature ofthe low-side switching device 3304 constituting the unit chopper cell3120.

The low-side switching device 3304 consists of IGBT 4000 and Diode 4001,and the IGBT 4000 and the Diode 4001 are fixed to the same cooling fin4002.

A heat P_IGBT and a heat P_Diode which are generated by the IGBT 4000and the Diode 4001, respectively, when the short-circuit current Ishflows are discharged into the air from the cooling fin.

FIG. 38 is an illustration that replaces a thermal circuit in thecooling feature of FIG. 37 with an equivalent electric circuit.

The heat P_IGBT generated by the IGBT 4000 and the heat P_Diodegenerated by the Diode 4001 can be expressed by a current source 4100and a current source 4110, respectively.

In addition, a heat resistance Rth(j−c)q and heat capacity Cth(j−c)qbetween the junction and case of the IGBT 4000 can be expressed by aresistor 4101 and a capacitor 4102, respectively.

In addition, a heat resistance Rth(c−f)q and heat capacity Cth(c−f)qbetween the case of the IGBT 4000 and the cooling fin 4002 can beexpressed by a resistor 4103 and a capacitor 4104, respectively.

In addition, a heat resistance Rth(f−a)q and heat capacity Cth(f−a)qbetween the cooling fin 4002 and the air can be expressed by a resistor4105 and a capacitor 4106, respectively.

In addition, a heat resistance Rth(j−c)d and heat capacity Cth(j−c)dbetween the junction of the Diode 4001 and a case of the Diode 4001 canbe expressed by a resistor 4111 and a capacitor 4112, respectively.

In addition, a heat resistance Rth(c−f)d and heat capacity Cth(c−f)dbetween the case of the Diode 4001 and the cooling fin 4002 can beexpressed by a resistor 4113 and a capacitor 4114, respectively.

In addition, if a temperature of the air is assumed to be constant, atemperature Ta of the air can be expressed as a DC voltage source 4107.

In addition, voltages of the capacitors 4102 and 4112 on the highvoltage side correspond to junction temperatures of the IGBT and theDiode, respectively.

Therefore, by lowering the temperature Ta of the air, the junctiontemperatures of the IGBT and the Diode when the short-circuit currentflows can be lowered below the predetermined value.

In addition, by decreasing the heat resistance Rth(j−c)q between thejunction of the IGBT 4000 and the case of the IGBT 4000, or bydecreasing the heat resistance Rth(c−f)q between the case of the IGBT4000 and the cooling fin 4002, or by decreasing the heat resistanceRth(f−a)q between the cooling fin 4002 and the air, the junctiontemperature of the IGBT when the short-circuit current flows can belowered below a predetermined value.

In addition, by decreasing the heat resistance Rth(j−c)d between thejunction of the Diode 4001 and the case of the Diode 4001, the heatresistance Rth(c−f)d between the case of the Diode 4001 and the coolingfin 4002, and the heat resistance Rth(f−a)q between the cooling fin 4002and the air, the junction temperature of the Diode when theshort-circuit current flows can be lowered below a predetermined value.

In addition, the short-circuit current flows during only a time(generally, several tens of milliseconds to several hundreds ofmilliseconds) until the power conversion device 3101 is disconnectedfrom the three-phase AC power system 3100 or 3170 by the breaker 3124.Therefore, by increasing the heat capacitance Cth(j−c)q between thejunction of the IGBT 4000 and the case of the IGBT 4000, or the heatcapacitance Cth(c−f)q between the case of the IGBT 4000 and the coolingfin 4002, or the heat capacitance Cth(f−a)q between the cooling fin 4002and the air, the junction temperature of the IGBT can be maintainedbelow the predetermined value during the time until the power conversiondevice 3101 is disconnected from the three-phase AC power system 3100 or3170.

In addition, by increasing the heat capacitance Cth(j−c)d between thejunction of the Diode 4001 and the case of the Diode 4001, or the heatcapacitance Cth(c−f)d between the case of the Diode 4001 and the coolingfin 4002, or the heat capacitance Cth(f−a)q between the cooling fin 4002and the air, the junction temperature of the Diode can be maintainedbelow the predetermined value during the time until the power conversiondevice 3101 is disconnected from the three-phase AC power system 3100 or3170.

In addition, in the present embodiment, the primary winding of thetransformer is delta-connected. However, the embodiment can also beapplied to other connections, such as, star connection.

In addition, in the present embodiment, a midpoint-grounded two-line DCtransmission method that connects two power conversion devices in seriesto respective sides of the DC transmission system and grounds theconnection points thereof was adopted. However, the embodiment may alsobe applied to other DC transmission methods such as, a two-line DCtransmission method that connects only one power conversion device torespective sides of the DC transmission system, and a midpoint-groundedthree-line DC transmission method that connects two power conversiondevices in series to respective sides of the DC transmission system,while grounding the respective connection points and connecting theconnection points to each other by a cable.

In addition, in the present embodiment, the explanation has been givenusing a DC transmission system as an example. However, the embodimentcan be applied to a power conversion device, such as, a reactive powercompensating device and a motor drive power conversion device thatconnect one end thereof to a three-phase AC power system and convert ACpower into DC power.

Thirteenth Embodiment

An explanation will be given of a thirteenth embodiment.

The thirteenth embodiment is identical to the twelfth embodiment exceptthat in the twelfth embodiment, a converter arm is configured with achopper cell, however, in the thirteenth embodiment, the converter armis configured with a unit full-bridge shown in FIG. 32.

Below, an explanation will be given of only a part of configuration ofthe thirteenth embodiment different from the twelfth embodiment.

FIG. 31 is a circuit diagram showing a thirteenth embodiment of thepresent invention. A power conversion device 3401 is interconnected withthe three-phase AC power systems 3100 and 3170 through three-phase ACterminals 3102 to 3104, and transmits and receives active/reactive powerto and from the three-phase AC power system 3100. The power conversiondevice 3401 consists of a transformer 3105, a positive converter group3112 and a negative converter group 3116.

Next, an explanation will be given of configurations of the positiveconverter group 3112 and the negative converter group 3116.

The positive converter group 3112 includes a u-phase positive converterarm 3113, a v-phase positive converter arm 3114 and a w-phase positiveconverter arm 3115. In addition, the negative converter group 3116includes a u-phase negative converter arm 3117, a v-phase negativeconverter arm 3118 and a w-phase negative converter arm 3119.

In addition, each of the converter arms 3113 to 3115 and 3117 to 3118 isa circuit which cascade-connects one or a plurality of unit full-bridgecells 3400 shown in FIG. 32.

The unit full-bridge cell 3400 is a two-terminal circuit having anx-terminal 3500 and a y-terminal 3501, and consists of an x-phasehigh-side switching device 3502, an x-phase low-side switching device3503, a y-phase high-side switching device 3504, a y-phase low-sideswitching device 3505 and an energy storage device 3506. The switchingdevices 3502 to 3505 are semiconductor switching devices represented byIGBT. In addition, the energy storage device 3506 is, for example, acapacitor or a battery. In the embodiment, a voltage of the x-terminalagainst a standard voltage of the y-terminal is also called a cellvoltage Vcell of the unit full-bridge cell.

Next an explanation will be given about that the arm voltage can becontrolled by a switching condition of the switching device constitutingthe unit full-bridge cell 3400.

The x-phase high-side switching device 3502 and the x-phase low-sideswitching device 3503 are alternately turned ON/OFF. In addition, they-phase high-side switching device 3504 and the y-phase low-sideswitching device 3505 are alternately turned ON/OFF.

When the x-phase high-side switching device 3502 is ON, the x-phaselow-side switching device 3503 is OFF, the y-phase high-side switchingdevice 3504 is OFF and the y-phase low-side switching device 3505 is ON,the cell voltage Vcell is substantially equal to a voltage VC of theenergy storage device 3506 without depending on the current Icell.

When the x-phase high-side switching device 3502 is ON, the x-phaselow-side switching device 3503 is OFF, the y-phase high-side switchingdevice 3504 is ON and the y-phase low-side switching device 3505 is OFF,the cell voltage Vcell is substantially zero without depending on thecurrent Icell.

When the x-phase high-side switching device 3502 is OFF, the x-phaselow-side switching device 3503 is ON, the y-phase high-side switchingdevice 3504 is OFF and the y-phase low-side switching device 3505 is ON,the cell voltage Vcell is substantially zero without depending on thecurrent Icell.

When the x-phase high-side switching device 3502 is OFF, the x-phaselow-side switching device 3503 is ON, the y-phase high-side switchingdevice 3504 is ON and the y-phase low-side switching device 3505 is OFF,the cell voltage Vcell is substantially equal to a reverse voltage ofthe voltage VC of the energy storage device 3506 without depending onthe current Icell.

When the x-phase high-side switching device 3502, the x-phase low-sideswitching device 3503, the y-phase high-side switching device 3504 andthe y-phase low-side switching device 3505 are all OFF, the cell voltageVcell is determined depending on a polarity of the current Icell. If theIcell is positive, the cell voltage Vcell is substantially equal to thevoltage VC of the energy storage device 3506. If the Icell is negative,the cell voltage Vcell is substantially equal to a reverse voltage ofthe voltage VC of the energy storage device 3506.

Next, an explanation will be given of operations of the power conversiondevice 3401 when the DC line is short-circuited.

As with the twelfth embodiment, since a protection operation of thepower conversion device 3401 for the short-circuiting of AD line isdifferent from the protection operation for the short-circuiting of DCline, it is required to distinguish the short-circuiting of AD line fromthe short-circuiting of DC line.

When the DC line is short-circuited, charges stored in the energystorage device 3506 in each unit chopper cell 3400 are discharged in theDC line, and the current ID increases. If a current censor 3123 detectsthe current ID and the current ID exceeds a predetermined thresholdvalue, it is determined that the DC line is in trouble, and the x-phasehigh-side switching device 3502 as well as the y-phase high-sideswitching device 3504 of each unit full-bridge cell 3400 are turned OFFand the x-phase low-side switching device 3503 as well as the y-phaselow-side switching device 3505 are turned ON, or the x-phase high-sideswitching device 3502 as well as the y-phase high-side switching device3504 are turned ON and the x-phase low-side switching device 3503 aswell as the y-phase low-side switching device 3505 are turned OFF. Adiode is connected in anti-parallel to the x-phase high-side switchingdevice 3502, the y-phase high-side switching device 3504, the x-phaselow-side switching device 3503 and the y-phase low-side switching device3505, respectively and the diode has a reverse blocking characteristicfor the current. Therefore, the energy storage device 3506 iselectrically insulated from the DC line, thereby, the overcurrent to theDC transmission cable 3150 can be suppressed.

In addition, when a current censor is set in the a-terminal or theb-terminal of converter arm of each phase, and if a sum of currents ofthe three phases flowing in respective converter arms exceeds apredetermined threshold value, it may be determined that the DC line isin trouble.

In addition, if it is determined that the DC line is in trouble, thepower conversion device 3401 is disconnected from the three-phase ACpower system 3100 or 3170 in a short time (generally, several tens ofmilliseconds to several hundreds of milliseconds) by the breaker 3124. Ashort-circuit current Ish flows from the three-phase AC power system3100 or 3170 depending on a leakage impedance of the transformer 3105during a short time until the power conversion device 3401 isdisconnected from the three-phase AC power system 3100 or 3170. If avoltage of the three-phase AC power system 3100 or 3170 is denoted by Vsand the leakage impedance of the transformer 3105 is denoted by Ztr, theshort-circuit current Ish is expressed by the following formula.

Ish=Vs/Ztr  [Number 4]

If a saturation current of a semiconductor switching device constitutingthe unit full-bridge cell 3400 is denoted by Isa, the semiconductorswitching device can be protected by adjusting the leakage impedance Ztrof the transformer 3105 so as to satisfy the condition of Isa>Ish.

In addition, as with the twelfth embodiment, the semiconductor switchingdevice can be protected by disposing a cooling system of thesemiconductor switching device so that a junction temperature of thesemiconductor switching device does not exceed a predeterminedtemperature during a time (generally, several tens of milliseconds toseveral hundreds of milliseconds) until the power conversion device 3401is disconnected from the three-phase AC power system 3100 or 3170 by thebreaker 3124.

In addition, if it is determined that the DC line is in trouble, in thepower conversion device 3401, since a sum of voltages of the positiveconversion group 3112 and the negative conversion group 3116 is madesubstantially equal to a voltage with reverse phase of that of thethree-phase AC power system 3100 or 3170, a voltage of the DC terminalcan be made zero. Accordingly, the current ID flowing in the DC outputterminal can be reduced.

In addition, in the present embodiment, the primary winding of thetransformer is delta-connected. However, the embodiment may also beapplied to other wiring connections, such as star connection.

In addition, in the present embodiment, a midpoint-grounded two-line DCtransmission method that connects two power conversion devices in seriesto respective sides of the DC transmission system and grounds theconnection points thereof was adopted. However, the embodiment may alsobe applied to other DC transmission methods such as, a two-line DCtransmission method that connects only one power conversion device torespective sides of the DC transmission system, and a midpoint-groundedthree-line DC transmission method that connects two power conversiondevices in series to respective sides of the DC transmission system,while grounding the respective connection points and connecting theconnection points to each other by a cable.

In addition, in the present embodiment, the explanation has been givenusing a DC transmission system as an example. However, the embodimentcan be applied to a power conversion device, such as, a reactive powercompensating device and a motor drive power conversion device thatconnect one end thereof to a three-phase AC power system and convert ACpower into DC power.

Fourteenth Embodiment

An explanation will be given of a fourteenth embodiment embodying thepresent invention. The fourteenth embodiment is identical to the twelfthembodiment except that in the twelfth embodiment, a secondary voltage ofthe transformer is applied within a phase, however, in the fourteenthembodiment, the secondary voltage of the transformer is applied betweenphases.

Below, an explanation will be given of only a part of configuration ofthe fourteenth embodiment different from the twelfth embodiment.

FIG. 33 is a circuit diagram showing the fourteenth embodiment of thepresent invention.

AC power from the three-phase AC power systems 3100 and 3170 isconverted into DC power by two power conversion devices 3601interconnected with the three-phase AC power system, and the DC power istransmitted in one direction or bi-directionally through the DCtransmission cable 3150.

The power conversion device 3601 consists of a transformer 3600, apositive converter group 3112, a negative converter group 3116, apositive reactor group 3602 and a negative reactor group 3603.

The positive converter group 3112 has a configuration that star-connectsa u-phase positive converter arm 3113, a v-phase positive converter arm3114 and a w-phase positive conversion.

The negative converter group 3116 has a configuration that star-connectsa u-phase negative converter arm 3117, a v-phase negative converter arm3118 and a w-phase negative conversion are 3119.

One terminal of the positive reactor group 3602 is connected in seriesto the b-terminal of the positive converter group 3112, one terminal ofthe negative reactor group 3603 is connected in series to the otherterminal of the positive reactor group 3602, and the a-terminal of thenegative converter group 3116 is connected in series to the otherterminal of the negative reactor group 3603.

In the specification, a circuit that connects one terminal of a positivereactor in series to the b-terminal of a positive converter arm, oneterminal of the negative reactor is connected in series to the otherterminal of the positive reactor, and the a-terminal of the negativeconverter arm is connected in series to the other terminal of thenegative reactor is called a leg.

The a-terminal of the positive converter group 3112 is called aP-terminal, a connection point of the two reactor groups is called anM-terminal and the b-terminal of the negative converter group 3116 iscalled an N-terminal.

With respect to the two power conversion devices 3601, the P-terminal ofone power conversion device 3601 and the N-terminal of the other powerconversion device 3601 are connected to the DC transmission cable 3150,respectively, and the N-terminal of the one power conversion device 3601and the P-terminal of the other power conversion device 3601 areconnected to each other and the connection point 3161 is grounded.

An explanation will be given of the positive reactor group 3602 and thenegative reactor group 3603 of the power conversion device 3601.

In the positive converter group 3112 and the negative converter group3116, since the voltage Vcell of the unit chopper cell is limited tomultiples of the voltage VC of the energy storage device 3305,instantaneous voltage values of respective legs are different to eachother.

In a period when leg voltages of three legs are different to each other,a difference of the leg voltage is absorbed by only the two reactorsincluded in each leg, and if the two rectors do not exist, anovercurrent flows in the legs.

The positive reactor group 3602 and the negative reactor group 3603 havea role to prevent the overcurrent.

Next, an explanation will be given about that an operation of the powerconversion device 3601 is different between the short-circuiting of ACline and short-circuiting of DC line.

When AC line is short-circuited, if the power conversion device 3601 isoutputting a voltage to the AC line, a short-circuit current flows.Therefore, when the AC line is short-circuited, as with the generalpower conversion device, the high-side switching device 3303 and thelow-side switching device 3304 both constituting each unit chopper cell3120 are both turned OFF in order to prevent the short-circuit current,and thereby overcurrent in the AC line can be prevented.

When the DC line is short-circuited, charges stored in the energystorage device 3305 in each unit chopper cell 3120 are discharged in theDC line, and the current ID becomes an overcurrent. In order to preventthe overcurrent, in each unit chopper cell 3120, the high-side switchingdevice 3303 is turned OFF and the low-side switching device 3304 isturned ON. A diode is connected to the high-side switching device 3303in anti-parallel. Since the diode has a reverse blocking characteristic,the energy storage device 3305 is electrically insulated from the DCline, and accordingly, the overcurrent flowing into the DC transmissioncable 3150 can be suppressed.

As described above, since a protection operation of the power conversiondevice 3601 is different between the short-circuiting of AD line and theshort-circuiting of DC line, it is required to distinguish theshort-circuiting of AD line from the short-circuiting of DC line.

When the AC line is short-circuited, a detected current value detectedby a current censor set in the primary winding side or the secondarywinding side of the transformer 3600 increases. Therefore, if thecurrent detected by the current censor set in the primary winding sideor the secondary winding side of the transformer 3600 exceeds apredetermined threshold value, it is determined that the AC line is introuble.

In addition, current censors are set in the positive converter group3112 and the negative converter group 3116 for respective phases, and ifa difference between current values detected by the current censor setin the positive converter group 3112 and the negative converter group3116 exceeds a predetermined threshold value, it may be determined thatthe AC line is in trouble.

On the other hand, when the DC line is short-circuited, if a currentdetected by the current censor 3123 set in the DC line exceeds apredetermined threshold value, it is determined that the DC line is introuble.

In addition, a current censor is set in the a-terminal or the b-terminalof converter arm of each phase, and if a sum of currents of the threephases flowing in respective converter arms exceeds a predeterminedthreshold value, it may be determined that the DC line is in trouble.

If it is determined that the AC line or DC line is in trouble, the powerconversion device 3601 is disconnected from the three-phase AC powersystem 3100 or 3170 in a short time (generally, several tens ofmilliseconds to several hundreds of milliseconds) by the breaker 3124.

Meanwhile, the present embodiment can also be applied to a powerconversion device interconnected with a single-phase or a multiphasesystem by increasing or decreasing the number of converter arms, inaddition to the three-phase AC power system.

In addition, in the embodiment, the primary winding and the secondarywinding of the transformer are both delta-connected. However, thepresent invention is not limited to the delta connection with respect toa winding feature of the transformer.

In addition, in the present embodiment, a midpoint-grounded two-line DCtransmission method that connects two power conversion devices in seriesto respective sides of the DC transmission system and grounds theconnection points thereof was adopted. However, the embodiment may alsobe applied to other DC transmission methods such as, a two-line DCtransmission method that connects only one power conversion device torespective sides of the DC transmission system, and a midpoint-groundedthree-line DC transmission method that connects two power conversiondevices in series to respective sides of the DC transmission system,while grounding the respective connection points and connecting theconnection points to each other by a cable.

In addition, in the present embodiment, the explanation has been givenusing a DC transmission system as an example. However, the embodimentcan be applied to a power conversion device, such as, a reactive powercompensating device and a motor drive power conversion device thatconnect one end thereof to a three-phase AC power system and convert ACpower into DC power.

Fifteenth Embodiment

An explanation will be given of a fifteenth embodiment embodying thepresent invention. The fifteenth embodiment is a modification of thefourteenth embodiment. The fifteenth embodiment is identical to thefourteenth embodiment except that, in the fourteenth embodiment, thepositive converter group and the negative converter group are configuredwith chopper cells, however, in the fifteenth embodiment, the convertergroups are configured with unit full-bridge cells.

Below, an explanation will be given of only a part of configuration ofthe fifteenth embodiment different from the fourteenth embodiment.

FIG. 34 is a circuit diagram showing a fifteenth embodiment of thepresent invention. A power conversion device 3701 is interconnected withthe three-phase AC power system 3100 or 3170 through three-phase ACterminals 3102 to 3104, and transmits and receives active/reactive powerto and from the three-phase AC power system 3100 or 3170. The powerconversion device 3701 consists of the transformer 3600, the positiveconverter group 3112 and the negative converter group 3116.

Next, an explanation will be given of configurations of the positiveconverter group 3112 and the negative converter group 3116.

The positive converter group 3112 includes a u-phase positive converterarm 3113, a v-phase positive converter arm 3114 and a w-phase positiveconverter arm 3115. In addition, the negative converter group 3116includes a u-phase negative converter arm 3117, a v-phase negativeconverter arm 3118 and a w-phase negative converter arm 3119.

In addition, each of the converter arms 3113 to 3115 and 3117 to 3119includes an a-terminal and a b-terminal.

In the specification, a voltage of the a-terminal against a standardvoltage of the b-terminal is called an arm voltage. In addition, each ofthe converter arms 3113 to 3115 and 3117 to 3119 is a circuit whichcascade-connects one or a plurality of unit full-bridge cells 3400 shownin FIG. 32.

Next, an explanation will be given about that an operation of the powerconversion device 3701 is different between a short-circuiting of ACline and a short-circuiting of DC line.

When AC line is short-circuited, if the power conversion device 3701 isoutputting a voltage to the AC line, a short-circuit current flows.Therefore, when the AC line is short-circuited, as with the generalpower conversion device, an x-phase high-side switching device 3502, ay-phase high-side switching device 3504, an x-phase low-side switchingdevice 3503 and a y-phase low-side switching device 3505, eachconstituting respective unit full-bridge cells 3400, are all turned OFFin order to prevent the short-circuit current, and as a result, anovercurrent in the AC line can be prevented.

When the DC line is short-circuited, charges stored in the energystorage device 3506 in each full-bridge cell 3400 are discharged in theDC line, and the current ID increases. If the current ID is detected bya current censor 3123 set in the DC line and the current ID exceeds apredetermined threshold value, it is determined that the DC line is introuble, and the x-phase high-side switching device 3502 as well as they-phase high-side switching device 3504 of each unit full-bridge cell3400 are turned OFF and the x-phase low-side switching device 3503 aswell as the y-phase low-side switching device 3505 of each unitfull-bridge cell 3400 are turned ON, or the x-phase high-side switchingdevice 3502 as well as the y-phase high-side switching device 3504 areturned ON and the x-phase low-side switching device 3503 as well as they-phase low-side switching device 3505 are turned OFF. A diode isconnected to each of the x-phase high-side switching device 3502, they-phase high-side switching device 3504, the x-phase low-side switchingdevice 3503 and the y-phase low-side switching device 3505 inanti-parallel. Since the diode has a reverse blocking characteristic, aDC voltage of the energy storage device 3506 is electrically insulatedfrom the DC line, and as a result, the overcurrent flowing into the DCtransmission cable 3150 can be suppressed.

As described above, since the protection operation of the powerconversion device 3701 is different between the short-circuiting of ADline and the short-circuiting of DC line, it is required to distinguishthe short-circuiting of AD line from the short-circuiting of DC line.

When the AC line is short-circuited, a detected current value detectedby a current censor set in the primary winding side or the secondarywinding side of the transformer 3600 increases. Therefore, if thecurrent detected by the current censor set in the primary winding sideor the secondary winding side of the transformer 3600 exceeds apredetermined threshold value, it is determined that the AC line is introuble.

In addition, current censors are set in the positive converter group3112 and the negative converter group 3116 for respective phases, and ifa difference between current values detected by the current censor setin the positive converter group 3112 and the negative converter group3116 exceeds a predetermined threshold value, it may be determined thatthe AC line is in trouble.

On the other hand, when the DC line is short-circuited, if a currentdetected by the current censor 3123 set in the DC line exceeds apredetermined threshold value, it is determined that the DC line is introuble.

In addition, a current censor is set in the a-terminal or the b-terminalof converter arm of each phase, and if a sum of currents of three phasesflowing in respective converter arms exceeds a predetermined thresholdvalue, it may be determined that the DC line is in trouble.

In addition, when it is determined that the DC line is in trouble, thepower conversion device 3701 outputs a voltage having a reverse phase ofthe voltage of the three-phase AC power system 3100 or 3170 in order tomake the DC terminal voltage be zero. Then, the current ID flowing inthe DC output terminal can be reduced.

When it is determined that the AC line or DC line is in trouble, thepower conversion device 3701 is disconnected from the three-phase ACpower system 3100 or 3170 in a short time (generally, several tens ofmilliseconds to several hundreds of milliseconds) by the breaker 3124.

Meanwhile, the present embodiment can also be applied to a powerconversion device interconnected with a single-phase or a multiphasesystem by increasing or decreasing the number of converter arms, inaddition to the three-phase AC power system.

In addition, in the embodiment, the primary winding and the secondarywinding of the transformer are both delta-connected. However, thepresent invention is not limited to the delta connection with respect toa winding feature of a transformer.

In addition, in the present embodiment, a midpoint-grounded two-line DCtransmission method that connects two power conversion devices in seriesto respective sides of the DC transmission system and grounds theconnection points thereof was adopted. However, the embodiment may alsobe applied to other DC transmission methods such as, a two-line DCtransmission method that connects only one power conversion device torespective sides of the DC transmission system, and a midpoint-groundedthree-line DC transmission method that connects two power conversiondevices in series to respective sides of the DC transmission system,while grounding the respective connection points and connecting theconnection points to each other by a cable.

In addition, in the present embodiment, the explanation has been givenusing a DC transmission system as an example. However, the embodimentcan be applied to a power conversion device, such as, a reactive powercompensating device and a motor drive power conversion device thatconnect one end thereof to a three-phase AC power system and convert ACpower into DC power.

POTENTIAL FOR INDUSTRIAL APPLICATION

The power conversion device of the present invention is applicable to,for example, a reactive power compensation device (STATCOM), aBack-to-Back system (for example, frequency conversion device), a DCtransmission system (HVDC) and a motor drive.

In addition, other than the DC transmission system (HVDC) that transmitselectric power by converting AC power into DC power once, the presentinvention is applicable to a power conversion device, for example, areactive power compensation device and a motor drive power conversiondevice that connect one end thereof to a three-phase AC power system andconvert AC power to DC power.

EXPLANATION FOR REFERENCE NUMBER

-   100, 500, 1600, 1601 Three-phase power system-   101, 600, 800, 1000, 1200, 1700, 1900, 2100, 2300, 2500, 2700 Power-   conversion device-   102 R-phase terminal-   103 S-phase terminal-   104 T-phase terminal-   105, 601, 801, 1001, 1201, 1701, 1901, 2101, 2301, 2501 Transformer-   106, 1202 u-phase positive terminal-   107, 1203 v-phase positive terminal-   108, 1204 w-phase positive terminal-   109, 1205 u-phase negative terminal-   110, 1207 w-phase negative terminal-   111, 1206 v-phase negative terminal-   112 Positive converter group-   113 u-phase positive converter arm-   114 v-phase positive converter arm-   115 w-phase positive converter arm-   116, 2701 Negative converter group-   117 u-phase negative converter arm-   118 v-phase negative converter arm-   119 w-phase negative converter arm-   120 Unit converter-   121 Positive output terminal-   122, 402 Negative output terminal-   200 Primary winding-   201, 700, 900, 1300, 2000, 2400 Secondary winding-   202, 203, 204 Iron core-   205 Winding between R-phase and S-phase-   206 Winding between S-phase and T-phase-   207 Winding between T-phase and R-phase-   208, 901, 2401 u-phase winding-   209, 902, 2402 v-phase winding-   210, 903, 2403 w-phase winding-   300 x-terminal-   301 y-terminal-   302 x-phase high-side switching device-   303 x-phase low-side switching device-   304 y-phase high-side switching device-   305 y-phase low-side switching device-   306, 405 DC capacitor-   400 Bidirectional unit chopper converter-   401 x-phase output terminal-   403 High-side switching device-   404 Low-side switching device-   501 Electric power substation-   502 Electric power substation busbar-   503 Load-   802, 1902, 2302 u-phase terminal-   803, 1903, 2303 v-phase terminal-   804, 1904, 2304 w-phase terminal-   805, 1905 Neutral point terminal-   1301 u-phase positive winding-   1302 v-phase positive winding-   1303 w-phase positive winding-   1304 u-phase negative winding-   1305 v-phase negative winding-   1306 w-phase negative winding-   1602 Submarine cable-   2404 Compensating winding-   2405 u-phase compensating winding-   2406 v-phase compensating winding-   2407 w-phase compensating winding-   3100, 3170 Three-phase AC power system-   3101, 3401, 3601, 3701 Power conversion device-   3102 R-phase terminal-   3103 S-phase terminal-   3104 T-phase terminal-   3105, 3600, 3805 Transformer-   3106 u-phase positive terminal-   3107 v-phase positive terminal-   3108 w-phase positive terminal-   3109 u-phase negative terminal-   3110 v-phase negative terminal-   3111 w-phase negative terminal-   3112 Positive converter group-   3113 u-phase positive converter arm-   3114 v-phase positive converter arm-   3115 w-phase positive converter arm-   3116 Negative converter group-   3117 u-phase negative converter arm-   3118 v-phase negative converter arm-   3119 w-phase negative converter arm-   3120 Unit chopper cell-   3121 Positive DC output terminal-   3122 Negative DC output terminal-   3123 Current censor-   3124 Breaker-   3134 u-phase positive winding-   3135 v-phase positive winding-   3136 w-phase positive winding-   3137 u-phase negative winding-   3138 v-phase negative winding-   3139 w-phase negative winding-   3150, 3807 DC transmission cable-   3161, 3806 Neutral point-   3200 Secondary winding-   3201 Primary winding-   3202, 3203, 3204 Iron core-   3301 x-terminal-   3302 y-terminal-   3303 High-side switching device-   3304 Low-side switching device-   3305, 3506 Energy storage device-   3400 Unit full-bridge device-   3500 x-phase output terminal-   3501 y-phase output terminal-   3502 x-phase high-side switching device-   3503 x-phase low-side switching device-   3504 y-phase high-side switching device-   3505 y-phase low-side switching device-   3602 Positive reactor group-   3603 Negative reactor group-   3604 Positive u-phase reactor-   3605 Positive v-phase reactor-   3606 Positive w-phase reactor-   3607 Negative u-phase reactor-   3608 Negative v-phase reactor-   3609 Negative w-phase reactor-   3800 DC transmission system-   3801 Three-phase full-bridge power conversion device-   3802, 3803 Capacitor-   3804 DC reactor-   3901 Connection point-   4000 IGBT-   4001 Diode-   4002 Cooling fin-   4100 IGBT heat generation simulated current source-   4101 IGBT heat resistance between junction and case-   4102 IGBT heat capacity between junction and case-   4103 IGBT heat resistance between case and cooling fin-   4104 IGBT heat capacity between case and cooling fin-   4105 IGBT heat resistance between cooling fin and the air-   4106 IGBT heat capacity between cooling fin and the air-   4107 Air temperature simulated voltage source-   4110 Diode heat generation simulated current source-   4111 Diode heat resistance between junction and case-   4112 Diode heat capacity between junction and case-   4113 Diode heat resistance between case and cooling fin-   4114 Diode heat capacity between case and cooling fins.

1. A power conversion device comprising a plurality of series circuitseach comprised of a voltage source and a controlled current source,wherein at least two of said series circuits comprised of the voltagesource and the controlled current source are connected in parallel, andparallel connection points of the series circuits connected in parallelform output terminals.
 2. A power conversion device configured byconnecting a circuit that star-connects three controlled current sourcesto respective phases of a three-phase voltage source from which aneutral point of the three-phase voltage source is drawn out, wherein aneutral point of the three controlled current sources and the neutralpoint of the three-phase voltage source form output terminals.
 3. Thepower conversion device according to claim 1, wherein the voltage sourcecontains only a differential mode (or normal phase/reverse phase)component, and the controlled current source transmits and receiveselectric power to and from the voltage source by controlling thedifferential made (or normal phase/reverse phase) component andtransmits and receives electric power to and from a load deviceconnected to the output terminals or a power source by controlling acommon mode (or zero-phase) component.
 4. A power conversion devicecomprising a signal-phase or multiphase wherein in the power conversiondevice, each phase of a primary winding of the single-phase ormultiphase transformer forms an input terminal; a neutral point is drawnout from a secondary winding of the single-phase or multiphasetransformer; series circuits of the secondary winding of the transformerand the converter arm connected in parallel; and a parallel connectionpoint of the series circuits and the neutral point of the secondarywinding form output terminals.
 5. A power conversion device configuredby connecting a three-phase transformer, where a neutral point is drawnout from a secondary winding of the three-phase transformer, and acircuit that star-connects three converter arms to respective phases ofthe secondary winding, wherein each phase of a primary winding of thethree-phase transformer forms an input terminal, and a neutral point ofthe three converter arms and the neutral point of the secondary windingform output terminals.
 6. A power conversion device interconnected witha three-phase power system through a transformer, wherein a primarywinding of the transformer is connected to the three-phase power system,a neutral point of a secondary winding of the transformer is drawn out,forming four terminals; a circuit that star-connects three converterarms is connected to respective phases of the secondary winding otherthan the neutral point; and a neutral point of the three converter armsand the neutral point of the secondary winding form output terminals ofthe power conversion device.
 7. The power conversion device according toclaim 4, wherein the converter arm transmits and receives an electricpower to and from a single-phase or multiphase power system connected tothe primary winding of the transformer by controlling a differentialmode (normal phase/reverse phase) current, and transmits and receives anelectric power to and from a load device connected to the outputterminal or a power source by controlling a common mode (zero-phase)component.
 8. The power conversion device according to claim 4, whereinthe transformer comprises a winding structure or a method that makes amagnetomotive force caused by a common mode (zero-phase) current flowingin the secondary winding be substantially zero.
 9. The power conversiondevice according to claim 1, wherein each of said voltage sources ofsaid series circuits comprises a secondary winding of a transformercoupled to a multi-phase power source, and wherein each of saidcontrolled current sources of said series circuits comprises a seriesconnection of unit converters connected to a corresponding one of saidsecondary windings.
 10. The power conversion device according to claim9, wherein each of said unit converters comprises a full bridgeconverter.
 11. The power conversion device according to claim 9, whereineach of said unit converters comprises a bi-directional chopperconverter.
 12. A power conversion device connected with a multi-phasepower system through a transformer, wherein said transformer includes aprimary winding including a plurality of single-phase primary windingsand a secondary winding including a plurality of single-phase secondarywindings, said power conversion device comprising: a plurality of seriescircuits each comprised of one of the single-phase secondary windingsand a series connection of unit converters, wherein at least two of saidseries circuits are connected in parallel to one another, and whereinconnection points of the series circuits connected in parallel formoutput terminals of the power conversion device.
 13. The powerconversion device according to claim 12, wherein each of said unitconverters comprises a full bridge converter.
 14. The power conversiondevice according to claim 12, wherein each of said unit converterscomprises a bi-directional chopper converter.
 15. The power conversiondevice according to claim 12, wherein the secondary winding comprises afirst phase positive winding, a first phase negative winding, a secondphase positive winding, a second phase negative winding, a third phasepositive winding, and a third phase negative winding, wherein the first,second and third phase positive windings are connected to a positiveconverter group comprised of said unit converters, and wherein thefirst, second and third phase negative windings are connected to anegative converter group comprised of said unit converters, wherein afirst series connection winding is formed by the first phase positivewinding and the third phase negative winding, wherein a second seriesconnection winding is formed by the second phase positive winding andthe first phase negative winding and wherein a third series connectionwinding is formed by the third phase positive winding and the secondphase negative winding.
 16. The power conversion device according toclaim 12, wherein the first, second and third series connection windingsare configured so that a magnetomotive force caused by a zero-phasecurrent becomes zero.
 17. The power conversion device according to claim12, wherein the secondary winding comprises a first phase positivewinding, a first phase negative winding, a second phase positivewinding, a second phase negative winding, a third phase positivewinding, and a third phase negative winding, wherein the first, secondand third phase positive and negative windings are connected to first,second and third series connections of said unit converters, wherein afirst series connection winding is formed by the first phase positivewinding and the third phase negative winding and is connected to a firstone of said series connections of unit convertors, wherein a secondseries connection winding is formed by the second phase positive windingand the first phase negative winding and is connected to a second one ofsaid series connections of unit converters, and wherein a third seriesconnection winding is formed by the third phase positive winding and thesecond phase negative winding and is connected to a third one of saidseries connections of unit converters.
 18. The power conversion deviceaccording to claim 17, wherein the first, second and third seriesconnection windings are configured so that a magnetomotive force causedby a zero-phase current becomes zero.